The cushion, or inner portion of the mattress, can be made from foam, cotton or down, depending on your preferences. The least expensive cushion is foam, and it is readily available at discount and department stores. However, you can also purchase foam mattress cores online or in specialty stores for a substantially higher price (see Reference 2).
Measure the size of your bed so you know how much foam to purchase. Most foam for beds in stores comes in pre-cut rolls you can trim to fit your bed. Three to four layers of foam are normally used when making a DYI mattress (see Reference 1).
It is common to place a mattress topper over the layers of foam to provide a smooth surface without seams on the mattress (see Reference 1). Mattress toppers are also available at discount and department stores.
Determine if you need a hard base for your mattress. Some bed frames have only a few slats that cross the frame and require a sturdy base on the mattress. This can be a concern if you are not using a box spring under the mattress. If this is the case, purchase a latex pad for the bottom of the mattress to add support and prevent the mattress from falling through the slats on the frame (see Reference 1). When using a latex pad, slide the foam and topper onto the pad, once you have cut the pad for the dimensions of your bed.
Determine if you want a plastic shell for your mattress, or one made of material.
For a plastic-type shell, purchase a zippered mattress cover at your local discount of department store. These come in all bed sizes and are usually used to protect the mattress. All that is required after you make your mattress cover purchase is to slide the cushion portion of your mattress into the zippered cover and zip the cover shut and your mattress is complete.
For a material, futon-type outer shell, purchase two flat sheets to fit your bed size. Use a sewing machine to sew three sides of the sheets together and then sew Velcro along the fourth side of the sheets to make the mattress shell close (see Reference 3). Then slide the cushion portion of your mattress into the shell cover and press the Velcro together.