How do I get from Custom ROM back to stock?

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http://forum.xda-developers.com – So I installed TWRP and am Using CM10 on my S2 Skyrocket and I am trading it to a buddy who just wants it plain factory stock with no CWM and no Custom Roms. How do I get back to that point? Also, after we trade do I have to go into an ATT store to get our services switched? Or do I call them? (HowTos)