Short example of the config file:
T1_ITEMS:
  APPLE-0: 4, 5
T2_ITEMS:
  WOOL-2: 2, 1
  POTION-16485: 2, 2

Format: <MATERIAL_NAME>-<META_DATA>: <SPAWN_CHANCE>, <MAX_SPAWN_AMOUNT>

First you need to find the item material you want at http://minecraft-ids.grahamedgecombe.com/
On that site they display the material name like this (minecraft:<MATERIAL_NAME>)
Say you want to add a baked potato, search on 'potato' and you see Baked Potato in the list, with (minecraft:baked_potato) below it. So in this case you need to use "baked_potato" (can use all caps or not, doesn't matter).

There's two exceptions.
Say you want to add Charcoal, search on coal and you see both Coal and Charcoal with the same material name of 'coal', and Charcoal has an extra number '1' behind its ID like this 263:1. To add Charcoal to the config, use the material name of 'coal', but add this '1' as META_DATA. So in the config it should look like coal-1 or COAL-1. If you want to add regular coal both 'coal' or 'coal-0' work.

Second exception is potions.
For potions go to http://minecraft-ids.grahamedgecombe.com/potion-calculator
Configure the potion you want to add, and use the Metadata number on the site as META_DATA number in the config. You can see that the material name is 'potion'. So for some tier1 unextended drinkable regeneration you will need to use 'potion-8193'.

And if it wasn't clear, spawn chance is the spawn chance for that item. This is relative to the other items. So if you only have two items in the tier1 list, if both their spawn chances are '1', they have both the same chance of spawning compared to eachother.

Max spawn amount is the maximum amount of that item it can spawn in one go, with a minimum of 1.