Trainer making or modding is serious stuff. Mostly hex editing and you have to have a debug bios to use XDK-- I mean.. the assitant that you use to work on them... *shifty eyes* I've said to much!
*dissappears*
Your best bet would be www.xbox-scene.com
Learn about memory modifications and about EvoX trainers at Evox's site. All I know is that trainers run in memory and memory editing can be very difficult if you don't know what you are doing.
Good luck bro.