I'm about to flash CM10 to my friends S III. However, if it doesn't work properly, I want to be able to get back to stock rom, so how would I capture it, as I have only found Verizon and A&T roms, and we live in finland, where's nothing like this.
Easiest way would probably be to use Clock Work Mod or TWRP or some other recovery to create a snapshot of his phone as it is now. Then you can simply flash back to his phone as it was.
This will back up his current ROM with all his apps, settings, etc. If you want a clean factory image, then do a factory restore on the phone and then back that up with a recovery.
You might want to do both:
Backup as is now, with apps, settings and all, then factory restore and backup again. This way you can restore either to his phone as it is now, or to clean stock. But that's up to you.
A very good tool for managing all this is JRummy's ROM Toolbox (there is a lite version also, not sure what is left out in the lite version).