Touhou 12.3 is an "expansion" of 10.5, Scarlet Weather Rhapsody.

1) Unrar all three parts into any folder.  Runs where ever placed; no install needed.

2) Run th123_update_1xx.exe, up to what ever version you want to patch to.  Must patch sequentially!  Be sure to patch the game's root folder, not the "th123" subfolder!

3) Open configex123.ini and at the top where it says [th105path], replace the folder path in the ini file to the location you installed TH10.5.  Doing this will allow you to use all of the SWR characters in Hisoutensoku vs play (otherwise you only get the five new TH12.3 characters).

4) Apply English patch if desired: SokuEnglishTranslation-v1_0.exe

5) Copy score123.dat into your game folder and overwrite the original one to unlock 4 of every spellcard for every character (or don't, and unlock them the "right" way by playing the game).