All I did was use saeedavril's rx3 files. I opened them up using rx3 Master and extracted the textures. Once I had them I simply used photoshop to create my own tracksuit. Once I had that done I saved them back into the rx3 files and renamed the files to my teams number and installed them like his own kits.
It's nothing complex but I wouldn't want to make a tutorial saying "take saeedavril's work and edit it", that's a little too disrespectful to him.
I appreciate the time and hard work it must have taken to make all of those bench players and so I'm not going to encourage people to use his work without giving him credit, I strongly suggest if you do make your own kits in the manner that I did that you give saeedavril credit for the original design and say you modified it