To flash, download your rom of choice to your phone, enter clockwork recovery, make a full system backup, wipe data/cache/dalvik cache (under advanced), select choose zip from sdcard, find the rom file you downloaded and install, then reboot. Viola, custom rom!
This part can be the most confusing. To download the rom to your phone, first download it to your computer, plug in your phone via USB, then transfer the .zip (DO NOT UNZIP) to your phone's sdcard (save it to the parent directory for easy access).
Once that is done, open ROM Manager, click Flash Clockwork Recovery, then click Reboot to Recovery. That will boot you into Clockwork. From there, you will want to make your Nandroid backup.
To do this, use your volume rocker to select "backup and restore" and use your power button to select it. (You will always use volume/power to navigate in Clockwork Recovery, unless you download one of the new "touch" versions, I prefer the original.)
Select "backup" and let the phone make a backup.
Next, select "go back".
Select "Wipe Data/Factory Reset".
Select "Yes" and let it wipe itself.
Select "Install from sdcard".
Next, select "Choose zip from sdcard".
Find your ROM, and select it.
Select "Yes - install" and let it install.
Select "go back".
Select "reboot system now".
Login to google account.
Reboot.
Enjoy!