White garbage bags also make good diffusers. If you use incandescent light, make sure your camera is set up with indoor light settings or all your artwork will look quite orange. It's simple enough to correct the color temperature later with your image editor (if your monitor is close to properly calibrated), but you'll get the best contrast if you do it right in the camera to begin with. If your camera wants to know the temperature of the light, an incandescent lightbulb puts out light at approximately 2600 degrees Kelvin.
Since most tripods cannot shoot straight down, you can set up your art on a small easel in order to get it parallel to your camera's sensor. If you need to stiffen it, a black foamcore presentation board costs less than US$2 at Office Depot or another such supply store. Use a bit of rubber cement on each corner of the piece to attach it to the foamcore and keep the piece flat. I use black rather than white because it's easier to crop the board out of the image that way.