Find an old window screen at a yard sale, in your garage or ask your friends if they can spare one. It should be at least 3 feet wide by 3 feet long, but it can be longer or wider as needed.
Wash the screen thoroughly so that your sweater doesn't get dirty while laying on it.
Place the screen up off the ground. Sit it on the backs of two chairs or lay it up off the floor or table on a couple of bricks or pieces of wood. You want air circulating underneath the sweater as well as on top.
Make your own flat sweater dryer by purchasing a length of plastic screen from the hardware store and stapling it with a staple gun to a frame you make out of wood.
Use lightweight 1-by-1-inch wood cut to the length and width you want for your frame. You can use any wood you have to make a frame, but 1-by 1-is light and easy to use and a good choice, if you're purchasing wood.
Nail the wood together so that it looks like a large picture frame and then roll the screen out on it and staple it to the frame.
Have someone hold the screen out tightly for you so that you can staple it.