Martha Stewart has some directions for how to make a monitor cozy from an old issue of Blueprint, which reminded me of the monitor cozies I created for my boyfriend's two monitors last year. Lest you get discouraged by how nice and fancy Martha's version is, just check out the ones I made on my own, with just a bit of quick measuring and side-seam sewing. I also added a lining as well as a decorative fabric, since, like Martha says, it adds a little bit of color to the room.
Of course, hers are made much more carefully, where I'm more of a sew-by-the-seat-of-my-pants kind of gal. For instance, she adds gussets, and I don't even know what a gusset is.