Help! .. I need to unroot my phone

I got a Sprint Galaxy S2 Epic Touch 4G. I currently have the CM9 on it.. I need to unroot it.

1. What is the most up to date instructions how to unroot.

2. Do I have to flash a stock rom before unrooting.. (or is it part of the process)?

I know this phone had issues with flashing roms when on ICS .. what do I need to know about that?