You go to the ingame PC strip editor, choose the 'import file' option, import the PNGs you've saved to a uniform slot, close the game, drag the uniform files in Documents > KONAMI > save into the PC source files folder, take an Xbox source file and put it into the Xbox source folder, rename both source files to the same slot number, decrypt the PC uniform file with w!LD's tool, click the PC strip convert file, rehash and resign the output, then put it on your USB to copy to your Xbox. It's pretty simple if you watch the tutorial video.
I've redone a number of emblems (the promoted Premier League teams, new Swansea and Liverpool emblems) a LOT of kits (internationals pretty much redone, all the Premier League kits so I can save into the slots I want) - it's very easy to do but time consuming to do a lot.