From my experience replacing cracked screens on the Epic...
1. Acquire a working unit from eBay for appox. $50.
On both units:
2. Remove back cover, battery, & SD card.
3. Remove the seven screws(#000 phillips) found on the black back housing , then gently pry around the edges to unclip it (bottom to top seems to work best & the buttons are going to fall out).
4. Unhook the antenna cable (only the motherboard side) and both flex cables attached to the motherboard.
5. Since the motherboard is now free, simply put your original one in the fresh unit and reverse the above process.
If you've obtained a phone in fairly good condition, there's simply no need to disassemble either unit any further than what i listed above. Your goal should simply be to swap the motherboard.
BTW...You should definitely hang on to the remaining carcass because it has a bunch of good parts on it. Ex: camera, audio jack, loud speaker/antenna/vibrator ass'y