inexpensive white muslin from Joann Fabrics--about double the total perimeter of the desk so I could gather the fabric. The muslin came 36" wide, which worked out perfectly to cover the height of the desk plus provide a 6" flounce at the top. Because I wasn't cutting the selvage edges of the fabric, I didn't bother with any hemming. I simply folded the fabric down as long as I wanted the flounce, pinned it in place, and sewed a 1" pocket.
The biggest problem I encountered was getting the desk skirt to stay around the desk without sagging. I wanted the skirt to be removable for washing, so I didn't want to glue or staple anything in place permanently. In the end, I threaded 3/4" elastic through the skirt pocket, using about 6 feet of elastic to go around the 8-foot perimeter of the desk. The skirt still sagged a little, so I tacked it in place using white thumbtacks and a small hammer. The tacks will make the skirt a little harder to remove and replace, but they keep everything secure and neat looking:
This was one of the biggest projects I've ever completed, and while it took a little troubleshooting to get just right, I'm happy with the end result. It's both pretty and practical, which is exactly what I want out of a desk!