If you use ROM manager you probably have a custom recovery, as you can't really use it without clockworkmod recovery installed.
If you have a custom recovery, all you need to do is find a ROM you want to install, download it to your sdcard and then follow my steps. Backups, then whatever directions the ROMs say. Usually it is something like, reboot into recovery, wipe data and cache, install *insert name of ROM*.zip from sdcard, reboot into Android.
[video=youtube;s3HCzX9YBz0]http://www.youtube.com/watch?v=s3HCzX9YBz0[/video] <-- that video is for a different phone but it shows the general idea.