Your steps are correct and I would add to the end of the list to remove the rooting as well.
Two sites that will provide the majority of the information you are looking for are Android Forums and XDA Developers
Video and Guide - HOW TO ROOT THE HTC DESIRE (UNREVOKED METHOD)
Video and Guide - HOW TO LOAD A CUSTOM ROM ON THE HTC DESIRE
In order to backup your stock ROM, after you root your phone and before you install a custom ROM you will need to install "ROM Manager" (available in the Market). After "ROM Manager" is installed and you start it you will need to install ClockworkMod Recovery (the first option in ROM Manager). Once the custom recovery is installed, look further down ROM Manager's menu and you will see an option to backup your current ROM, which is what you want to do.
Once the backup is made copy the file to your computer so you have a secondary backup. In fact, copy the entire contents of your SD Card to your computer because sometimes the card is formatted or wiped during the install of some ROMs.
That will get you started but definitely do a lot of research so you understand what you are doing and more importantly what options are available. There are probably 30 ROMs available for the Desire and they all offer something different. You'll have to find the one that you like the most in order to get the best experience.