basically you need the full SBF first, search around here or google it and you'll find it online to download. then get the RSD lite program and reflash the phone. voila. the full SBF for 2.2.1 is called "VRZ_MB810_2.3.34_1FF_01.sbf", i googled it and found it online easily. it's around 445mb.
the one thing i found is that when i went from GB back to froyo i was stuck in a boot loop. go into recovery mode and do a factory wipe and you should be up and running with no issues after that. the tricky thing about recovery mode though was that when i depressed the camera button i got a reply "cannot find recovery file" or something like that. if i just lightly toggled the button it would select the option just fine (like when activating the autofocus in camera mode, vs. fully pressing and taking the photo).
just be sure to back up your stuff before hand! the whole process took me maybe 20 minutes from download to reboot.
Bob