Two or three users have had an issue flashing the backup after creating a restore. This seems to be caused by errors in Koush's clockworkmod recovery. That said, using clockworkmod is currently the only way to create a nandroid backup.
As always (and as stated in the forum guidelines) no one is responsible for your phone but you. This is where you should start in the Android Modding world and if you have problems then remember that any time you mess with /system files it can end in tears.
The first thing you should do after you root your phone (as in before doing anything else) is create a backup. This way you have somewhere to return to if you mess something up on your phone.
I read somewhere once that it's like having 999 lives in Contra. So true. So, I'm going to help you do it.
There are three tools you'll probably want. One is absolutely necessary, the other two are just handy to have.
Recommended (search app market, both have free versions):
- Titanium Backup
- Rom Manager