Once you've rooted, you install a custom recovery image that will allow backup. You should backup immediately after the root and recovery image is loaded. Should you ever need to go back to your stock rom there is a utility floating around from HTC/Sprint that will allow you to go back to stock, otherwise if you load a rom and don't like it you can always restore your first backup.
The actual flashing a rom process is silly easy. Once you've rooted you boot into the recovery image and choose to to and update from a specified zip file. The rom image will be in a zip file on your SDcard because you put it there. You point it to that and it does the process for you and reboots.
If something goes wrong you can end up with a non-working device so do this at your own risk as always.