How should I go about trimming a confederate rose bush?

The best way is to cut them back 1-2 feet in the fall after they bloom, but before the first freeze. You can then put the trimmings in a bucket of water and keep them in a dark place (like a garage or basement) for the winter. Just make sure they don't freeze. Then the trimmings can be planted in the spring to grow another bush if you want to!