Your best bet for a swift and relatively cheap solution, is an Xport for 360 and the PESRepublic or PESFan OF (or indeed both).
Those OFs have got a lot of the images on (they're passable on the PESREpublic one and good on PESFan), so you can then use those as a cut-and-paste base for all the tweaks you REALLY want in the in-game pixel editor (just Draw From Scratch, then go to Images and load the one you want, you can stretch and move it any way you want then).
But in order to get the UNIFORM files onto the HDD (or MU) you need the Datel 360 Xport (or Xsata, or Transfer Kit).
It's pretty simple, and you don't have to mod or hack the 360 at all (just put the HDD/MU in a USB docking station).
The UNIFORM files are .bins so they can't be viewed or created using the 3rd Party editors at the moment - and at the moment there's also no way to merge (or load/save part data from/to) the OFs; so if you've done some player edits and then you add stuff from the OF, you might lose your edits.
e.g. I have a Saints squad in Other Leagues B Slot A, but I can't get those players into the OF along with the full Prem and Spanish and National uniform files cos they started off in different OFs. And I can't get the emblem (rather than shirt front) out of the one OF and into another one either.
Basically you can port the Uniform files wherever, but can't get at the emblems or player data.
To my knowledge...