If you don't have one, use a web-based utility such as Pixlr, which is free and similar ... or a music lover who enjoys making custom mix-CDs with their own personalised covers, you could spend many ...
Once it's designed, simply print the CD cover on regular paper, cut it out and insert it into the CD case. CD covers can be fun to make on your own and are a great homemade gift to go along with a mix ...