I grew up in family fabric stores. Recovering valances/cornices is an easy project. You can probably order the same fabric you ordered your new drapes in if you want them to match. Coordinating is good, too, though, and you might find some really nice fabrics.
Go to any good fabric store that carries upholstery fabric. Chain fabric stores aren't a good bet - they carry seconds and cheap quality goods made for them. Fabric can suffer from dry rot, too, so choose carefully.
Take down the largest one of the cornices and carefully remove the staples. Determine whether or not you can use the same wood - more than likely you can. You can then evaluate the foam padding. New padding shouldn't cost too much - they use quilt batting for most window treatments. You can get it in packages at WM or another discount fabric place. It's called Mountain Mist and comes in different weights. Get the one with the "puffy" factor that you prefer, or re-use the existing padding. If it's foam rubber, plan on replacing, as it rots over time.
Use the dimensions of the largest cornice fabric piece to determine how many yards you need. Upholstery fabric comes in 54-60" widths, and you want to cut all the fabric on the straight grain. Cutting on the bias will stretch the fabric if you're not careful and makes it hard to get a nice taut finish.
If you don't have one, get a heavy duty stapler with the same size staples as what you took out - probably 3/16". Get spray adhesive, too, if you want the job to be easier. Use the spray adhesive to glue the batting in place. Then cover the cornice, stapling the fabric at the corners first, then the middle of each board, and then working around the edges to attach it all. You can trim some around the corners to get knife sharp edges if you want them.
Re-attach the hanging hardware and you've got a whole new look. It it really an easy project and you'll enjoy the results.
2014 Thor Motor Coach ACE 30.1 (The Mothership)
2013 Honda CR-V (The Pod)