I have only ever edited an existing player (i.e picking a very poor youth player you have never heard of who is unlikely to ever make it) at the club I want the playwer at (I make myself, albeit slightly younger and exeptionally better) Just go through page by page and change what you need to. I would of thought there wouldf be some You tube video kicking around for step by step.
I also prefer to make things a little more realistic, albeit still cheating, by picking some good youngsters, and have them on Loan for the Team I plan on Using.
E.G Danny Wilson (LFC) at Hartlepool - Not too unrealstic, but still a little stretch.
Also when you save the file after makign the changes, when you go to start your game, you need to tick the box that says use other Database files, or something along those lines.
Sorry it is a little vague, but to go through step by step would make for a huge post. hope it does shed a little light.