This is a tutorial on how to add custom cards to YGOPro. It may be complicated for the first time but with some practice it'll become a lot easier.
For safety measures create a COPY of YGOPRO folder and work in the copy, because if you do something wrong then it won't work.

Good idea publishing this to everyone, Tweedle. As I mentioned before this is really helpful

you could also use the dev pro card manager to add cards. It works for percy as wll as dev and tdaone.