Hisoutensoku netplay checklist:

1. Do you have all 20 characters unlocked? (Go to practice or Vs.Player and check)

This means linking Touhou 12.3 with touhou 10.5. To do this, open the configex.ini file in your touhou 12.3 folder, and you'll find something like this:

[th105path]
path=

Next to "path=" type in the directory to your touhou 10.5 game, (ex. path=C:\Games\th105)

2. Is your game updated to 1.10a?

This step is skippable, but if you do not have version 1.10a, you will be unable to run soku roll, which is highly recommended to reduce lag. Version 1.10a is compatible with 1.10.

Patch downloads: http://www.tasofro.net/touhou123/download.html
English patch: http://www.gensokyo.org/english-translations

A very common problem with the 1.10a patch is that it applies to the wrong directory. This is because when you pick a directory for the patch to apply to, it will add “/th123” at the end, causing a folder called “th123” to appear in your Hisoutensoku folder and doing absolutely nothing. This can be solved easily by removing the “/th123” at the end when it asks you for a directory to patch to.
Before: http://gyazo.com/0139495aba720ec1cdcfad8f8fcca10
After: http://gyazo.com/cca2bcb5b540d33f6e22f8c5f5adb82d

3. Do you have Soku roll 1.3 available?

This step is skippable, but it is HIGHLY recommended that you get soku roll in order to reduce the lag of long distance matches.

Soku roll is a program used to improve netplay by a considerable amount, you can download the current version (1.3) from here: http://is.gd/pjQZBI

If your game is correctly patched to 1.10a, simply run soku roll while Touhou 12.3 is running, and that's it! If an error occurs, a black box will pop up to tell you what's wrong, otherwise it'll flash very quickly on your screen. The name of Hisoutensoku's game window should change as well. Keep note that games running soku roll cannot connect to non-soku roll games and vice-versa.

4. Are firewalls or other programs preventing connection?
Something a lot of people neglect. If you have an anti-virus that comes with a firewall, be sure to make an exception (Or turn it off) for Hisoutensoku. Some programs (Like Norton) sometimes think that Sokuroll is a virus, and will block your connection. If you're skeptical about the soku roll program, you can ask in forums, chat rooms, or anywhere to confirm that it's a safe program. Even without an anti-virus, windows may have its own firewall, which you can change in the control panel of your computer, though the option to change it should come up when you try connecting anyways.

OPTIONAL: Are your ports forwarded?

This option is for those who wish to host games. Many players are confused on why they can't join their friends, and this is the reason why. Port forwarding allows players to create a “Virtual server”, which is necessary for allowing people to join your games. Port forwarding can be a big pain to do for the first time, but only one of the two players needs to do it in order to play.

Yaffy's fast guide to port forwarding (Should work for most routers, but not all): http://pastebin.com/zBBYGUnk

(If the guide above does not work, you will have better chances going through here: http://portforward.com/ )