Decide what size you want the futon to be. Futons can be the size of loveseats, or they can be the length of a full couch. Smaller loveseat futons combine two small mattresses to make one large mattress for sleeping, while larger futons have just one mattress.
If you want to build a full-size futon, plan on frame dimensions of 81 inches wide, 38 inches deep and 38 inches high. Mattress dimensions should be 54 inches wide and 75 long, with the thickness to your preference. Queen futons should be 86 inches wide, 38 inches deep and 43 inches high, with mattress dimensions 60 inches wide and 80 inches long. You can easily adjust these dimensions based on your size--petite individuals may prefer lower seat heights, for instance.
Decide if you want to use wood or metal for the frame. There are advantages and disadvantages to both of these materials. Wood is very heavy and is susceptible to insect damage (e.g., from termites), but it has a very warm feel and is quite durable. Metal will greatly lighten the frame weight, is just as durable as wood and has a more modern look, but rust can be an issue. If you plan to move a lot, metal probably is the better choice because it takes up less room when disassembled.
Assess your mattress comfort needs. Some people find that a thicker mattress feels too engulfing or doesn't support as well, or that it is too hot. If this is you, then limit the amount of foam or cotton stuffing you use. Other people find that a thick, cozy mattress is perfect; use more stuffing or foam in this case. The average thickness is six inches, with four- and eight-inch mattresses also available commercially.
Both the mattress and the cover should be washable, since it is inevitable that dust, dirt, body oil and food/drinks will get onto the cover and mattress. Cotton is the traditional material used for futon mattresses because it breathes better, therefore eliminating odors and the possibility of mold. Depending on your tastes, however, you can use other materials, especially on the cover, which can be made from alternative synthetics like polyester or acrylic.