patching on the xbox is not so easy as you are limited to the 8gb space on a DL disk unlike on a pc where hdd space is much greater. I have dabbled a bit over the years and found it easiest to modify other patches to my own tastes, good xbox patches to look out for are ones from bolloxmasta and wemexico. On pes 2012 I took the wemexico patch which had all the teams with correct kits and emblems as well as the mexican league and replaced the mexican teams with the npower championship teams. To modify any patch you first need a flashed xbox to play backup disks and an image of pro evo. Use a program called game graphic studio to open the pro evo image and you will see all the graphic files from the game, (just google pes 2012 afs map to see a list of all the files) from here it's a case of downloading new kits, emblems and whatever graphics you want and copying them over the original files.
If you want to create your own patch or make more deep modifications the you will need a program called afs explorer to extract files and then a hex editor like winhex to hex edit league structures, teamnames, player names and kit locations. There are lots of pitfalls, I have made many coasters going down this route that's why i find it easier to modify the great work of other patchers. also konami change the structure each year so making patches is similar but never the same year to year