Installing stock JellyBean in Galaxy S2 [Argentina]
You can get an almost stock Jellybean experience using SuperNexus - I9100. It is based off AOSP and only has "improvements and optimizations", but states it is still very near to stock. Another option, while differentiating itself from AOSP, is CyanogenMod. This is based on AOSP, but includes a LOT of extra features and improvements of AOSP, while keeping the bloat to a minimum.

As for instructions on getting it ready for a custom ROM, the instructions on the CyanogenMod wiki tend to be fairly accurate and may give you the needed information to get you started. Install CM for i9100 If you don't want CyanogenMod on your phone, replace step #2 in the last section with whatever ROM you want to try (like the one above). Make sure you get the right gapps (basic Google apps including the Play Store) unless your ROM includes it (most don't).