1 -
you can get the assets directly ingame by extracting the archive.rpa file ( i use unren, but they are others tool. unren is somewhere around here in f95, created by
@Sam if i remember good, maybe they are another better tool, i dont know). once you extracted the file, you got access to all the assets.
for myself, i got access to assets directly by AKABUR himself. i sadly cant share all my assets because i have some "hidden" stuff that AKABUR didnt have released yet, soo i avoid any possible spoil or troubles for AKABUR. in my assets i use the same as you can access in the rpa archive, but in high resolution photoshop sprites.
2-
check the "read me" or similar txt file in the download , that show how to install normally...
once installed, you have to browse througth the outfit to find the new replaced one. for now, we can't add new outfits directly in the game and need to "replace" existing outfit.
each numbers equal a outfit and his variations, for exempl, if April reporter outfit is b02, then b02b gonna be unzipped, b02c gonna be topless and etc...
i dont think they are a way to add ALL the outfit in the game, soo you need to replace some with the ones you want.
b01 is different than b02 and b03.. each outfit goes with his number and variables
b01 (standard) , b01b (standard topless),etc..
b02 (pyjama, b02b (pyjama topless) ,etc...
and so on...
its complicated to explain how to mod. you need the base sprite of the char ( nude body ), and the sprite of the outfit you want to add (or create, for that, you need to create an new sprite from scratch).
then you modify or adapt the outfit sprite to the body sprite.
you can technically do modding just like that. but if you want to go deeper like creating custom outfit or transfering one outfit from Ashelin to hermione (for exempl), you need to have knowledge and be ok to use photoshop, because its more complicated to adapt outfit, you need to fix/change alots of part of it without making it "weird" looking.