Backup whatever you have.
Download the one you like.
Drag it to the SD card once mounted on your computer.
Unmount SD card
Turn Phone OFF
Hold VOL DOWN and hit power, should boot in recovery, press home
Flash Zip and it should appear in the menu, if it doesn't its because its not in zip format (won't be if you used safari to download it) or its not in the right place. it has to be a free standing file can can't be in any other folders on the SD card.
Hit home to confirm.
Sit and wait, may take up to 10 minutes to install.
If you are installing a 2.1 ROM, Wipe Data/ factory reset prior to flashing the ROM.
If you are still having problems, I'll have GTalk on and just hit me up there: pharmerandy2010 AT gmail.com
Can I flash this rom to my phone if it has not been rooted.
No, you must Root the phone first. I wouldn't run the first option on gbhil's 2.1 leak post either, as its a pain to get back so that your phone will be updatable if you wish to have an official release or if your phone needs service.