Stand four pieces of wood, each 4-by-4-by-6 inches, on a flat surface. These will be the futon legs.
Lay a piece of wood, 39-by-75-by-2 inches, on top of the futon legs. This will be the surface that supports a twin-size futon mattress. Move the futon legs beneath the mattress support so that they hold up the mattress support by its corners. The end result should resemble a table.
Hammer 1.5-inch nails through the mattress support to secure the legs beneath it. To do this, stand a nail on top of the mattress support, over a corner of the futon leg, and hammer it straight down. Do this over each corner of each futon leg, using 16 nails.
Stuff a zip-up cotton mattress cover with cotton mattress pads. This will be the futon mattress. Since futons aren't as thick as traditional American mattresses, you need only enough padding to make the mattress 2 inches thick.
Zip the mattress cover. Place it one top of the futon.