This post was originally posted on my blog. It is very long and contains all of the main gold making methods that are relevant in Legion that I am aware of. Let me know if I have forgotten any and I'll look to adding them. You can find the original here:

http://thelazygoldmaker.com/legion-gold-guide

In Legion there are a ton of different ways to make gold. Navigating the jungle and finding out which methods are good for you and how to get started can be an extremely daunting task. In this Legion Gold Guide I will help you through all of the main gold making methods in legion and point you in the direction where you can learn more about them.
The foundation

To make money in WoW you need to have items that other people need or want. In WoW there are two primary categories of items that people are willing to pay for:

    Performance increasing items
    Cosmetic items

People are generally more willing to pay for performance increases than cosmetics, as it will directly impact the core gameplay. Performance increasing items are items that directly increase the power of your character. This can be gear, item enhancements, consumables etc.

Cosmetic items include mounts, transmog, rare recipes etc. These items don't increase your performance, but they are in demand because they look cool, give bragging rights etc.

After identifying items that people want you need to figure out how to obtain them at a lower price than the current auction house price, as this will ensure a profit. In general there are three ways you can obtain items in the game: buying them off the auction house, looting them (from mobs, nodes or similar) or crafting them. These methods help you obtain the items that you want to sell. This leads into the three main gold-making methods in WoW. Crafting, Farming and Flipping. So let's dig into them.
Crafting

Crafting is the art of buying materials and turning them into other materials or finished products that you can sell on the Auction house for a profit. It is one of the most consistent gold making methods in the game, but generally requires you to bind up some amount of capital in your inventory in the form of materials and finished items before they sell.
Alchemy

Alchemy is a crafting profession that can create potions and flasks, as well as some trinkets and mounts. The primary materials used are herbs. For potions and flasks only the Legion crafts have substantial demand as they are the only items that give meaningful performance increases at max level. Old world alchemy recipes that are profitable include Vial of the sands and transmutes.

Alchemy is a great profession for gold-making as the main outputs are consumed on use, this means that there will always be demand for new potions and flasks. The old world recipes are also very profitable. Sadly I don't have Legion alchemy so I can't personally help you. The best guide I was able to find on the internet is this video guide by Ninja Kuma.
Enchanting

Enchanting is a crafting profession that allows you to enchant gear with stat increases. It also allows you to disenchant gear for crafting materials. Enchanting uses crafting materials that can only be obtained from disenchanting gear. This means the only source is either crafting yourself or buying from other players. As the profession mostly revolves around stat increases only the legion recipes are relevant. Enchanting is also a great source of obliterum. The other ways to make gold with the profession is crafting and selling enchants and doing the enchanting shuffle.

Enchanting is one of the best crafting professions for making gold as the enchants are consumed on use, meaning that demand will never disappear.
Engineering

Engineering is a crafting profession that primarily revolves around fun recipes. The gold-making potential from engineering is limited in terms of Legion recipes. The Skullblasters can be obliterated for profit. Engineering has a lot of profitable old world recipes, primarily mounts and various unique effect items and toys. This profession is best relegated to an alt unless you love the lore-aspect or have other sentimental reasons to keep it on your main.

Engineering is not particularly strong from a gold-making point of vies, the old world mounts are great, but the Legion recipes are lacking and limited.
Jewelcrafting

Jewelcrafting is a crafting profession that revolves around gems. You prospect various ores turning them into gems and cut the gems. The cut gems can be added to socketed items. In addition you can craft rings that can either be disenchanted or sold as gearr. The Legion epic and rare gems and rings all sell very well. The jewelcrafting panther mounts are the best old world recipes.

Jewelcrafting is a very stronggold-making profession. Prospecting is often profitable and you can move a large amount of materials. For more details check out some of my posts on the matter or this Video Guide by WTBGold which covers all the shuffles in Legion.
Cooking

Cooking is a secondary crafting profession, which is available in addition to your two main professions. It takes various food crafting materials such as meats and fish and turn them into consumable food. The consumables give statbuffs and are used by raiders and other players seeking to maximize performance. It is a great money maker and you should have it on your main.
Blacksmithing

Blacksmithing is a crafting profession that uses bars obtained from melting ore to craft plate armor and weapons, as well as some assorted cosmetic items. In Legion you craft ilvl 850 gear (demonsteel), a mount and a legendary. These are the main gold making opportunities with blacksmithing. Blacksmithing can also create a lot of transmog gear from old expansion. Some rare recipes have looks that are very sought after primarily for weapons.

Check out my crafted transmog guide to test the transmog market, or you can copy my ilvl 850 setup from tailoring and use it with the blacksmithing gear.
Tailoring

Tailoring is a crafting profession that turns various forms of cloth into cloth gear. You can also create some tailoring only mounts. In Legion therer are two main gold-making opportunities, shuffling cloth into enchanting materials with the rank 3 silkweave bracers recipe and crafting ilvl 850 imbued silkweave gear to  sell on the auction house.

Old world recipes that are still relevant include transmog gear, old world enchanting shuffles and hexweave bags.
Leatherworking

Leatherworking is a crafting profession that uses various leathers obtained from skinning to craft leather and mail armor. You can make gold shuffling leather into enchanting materials through the rank 3 bracer recipes. Battlebound for Stormscale and Warhide for Stonehide Leather. The other main market for Leatherworking is crafting the item level 850 gear. I find that both Dreadleather and Gravenscale gear sells very well.
Inscription

Inscription is a very strong gold making profession. It utilises herbs and turn them into pigments for inks. The main outputs are glyphs that give cosmetic effects for your character. You can also craft trinkets that are upgradable with obliterum and Tome of The Tranquil Mind. As with Jewelcrafting you can make gold just by milling herbs. The glyphs aregenerally profitable, even though competition is usually high. The Darkmoon Deck trinkets have been amazing and so can Tome of the Tranquil Mind be.
Farming

Farming is all about going out into the game world and acquiring materials and items by interacting with it. There are two primary ways of generating items from the game world: Gathering from nodes using gathering professions and looting dead mobs.
Mining

Mining is one of the three gathering professions in the game. You mine from ore deposits for all the various ores in the game. Your customers will be blacksmiths, engineers and jewelcrafters. You can make good gold per hour from tons of different routes both farming for legion ores and farming old world ores. I personally don't farm, so I can't tell you the best routes, but check out Oldbess guide on mining for some great spots.
Herbalism

Herbalism is the other gathering profession in WoW. You pick flowers to gather herbs. Your main customers are alchemsits and scribes. I suggest sticking to legion herbs as demand for old world materials is very low for herbs. Ninja Kuma has you covered with great routes for all the Legion herbs.
Skinning

Skinning is the third gathering profession and it is very different from the other two. It works by skinning the dead bodies of animals with skins. You need to both kill the mob and then gather the body. Your main customers will be leatherworkers. Skinning in Legion is very good and gives a large amount of Blood of Sargeras. I suggest pairing this with a crafting profession that can unload a lot of bloods, like Alchemy or enchanting. For an overview of the best farming spots in Legion you can check out this guide by hikons or this one by WTBGold guides.
Raw gold

Raw gold farms revolve around farming areas where the mobs drop a lot of gold and you also get items you can sell to vendors. The primary goal is to just generate pure gold. The most popular raw gold farm is to run all the Cataclysm raids at 25 heroic. This will net about 1.5-2.5k gold per raid, and a chance at some rare mounts. Raw gold farms are great if you don't like interacting with the AH, dislike variance or you are in the process of building up your capital.
Specific items

The last group is the most diverse one. This encompasses all farms where you kill mobs looking for specific drops. The drops can be trade skill materials like volatile fire or highly sought after transmog pieces (AQ20) or a combination.

The youtuber Oldbess has a ton of farming videos out that cover all forms on farming and I suggest checking him out for some ideas on good spots.
Flipping

Flipping is the least time consuming way to make gold in WoW. It simply involves trading on the auction house. You buy low and sell high. There are a lot of different markets you can try your hand in flipping both Legion and old world. Flipping works best when there are natural variations in either demand or supply for an item which will cause the price to vary over time.

In Legion the best flipping markets are Epic Bind on equip items (BoEs) and materials, you can check out my writeups on flipping for more specific help getting started.

In old world markets you can try your hand at transmog, or trade skill materials. Check out my post on mining for an example of how you can flip old world materials (just make sure theres an actual market for the items you are looking to sell.
Useful Tools
TSM

The main tool for any serious gold maker is TradeSkillMaster. It is an addon suite with a desktop application that downloads price data and allows you to create rules for easy crafting, mailing, shopping and posting your items on the auction house. It has a premium service that includes e-mail notifications for great deals and importing great deals as a shopping scan.

TSM has a very steep learning curve, as it is a total profession framework. ITtdoes not hold your hand, and requires a lot of setup. Luckily you can import settings from other players and there are a ton of settings out there. I have published most of my settings in my pastebin here. You can also check out Sheyrahs. If you want to learn it from the ground up I suggest one of the following guides: PhatLewts, Sheyrah, WTBGold.
A second account

Once your auctioning operation reaches a certain size a second account becomes an invaluable tool. It will allow you to run auctioning scans, shopping scans and collect mailboxes while playing your main. I strongly suggest buying a second monitor as well. This will help you really unlock the benefit of a second account. For a detailed guide on why and how to set it up check out my post on the subject.