Step 4: Paint it!
If you feel that you don't need to use the space underneath the washer and dryer, just measure and cut a piece of plywood and nail or screw it in place. That size would be 15" x 27" (in my case). After that, You are are done! You've basically have made a platform!
If your washer and dryer are inside your house, instead of painting the wood, you might want to consider carpeting the top and side panels. This should help minimize the noise radiated from the washer onto the wood. You can also, instead of painting, cover it with some kind of vinyl, or tolex. Have fun and be creative.