If you need a new mattress but don't have the resources to buy one, it would help to know how to make a mattress from foam. When you open a mattress case, you can see that expensive mattresses are often only layers of foam. It is something you can easily do for yourself with or without a mattress case.
Measure your bed frame's width and length. The foam might come larger, but that is acceptable because you can cut it down to fit. Measure the height of the area where the mattress will go. Make sure that 14 inches of foam will fit.
Buy two layers of memory foam. Get the solid foam and not the egg-crate pattern. If this mattress is for a lightweight person, one layer might be plenty.
Buy one layer of extra-firm foam. If you have a deep space, you can use two layers. It is a firm support and gives enough of a base for the memory foam on top.
Allow the foam to uncurl and relax for several hours before putting it on the bed. Give the room plenty of ventilation to air out the smell of the foam.
Prepare the base by placing a solid piece of plywood on the base of your bed frame. If you are using a box mattress, you can skip this step. Half-inch thickness is usually plenty, but if this is a large bed, a 3/4-inch base might be more suitable.
Remove the mattress cover from your previous mattress. Unzip the cover and lay it on the plywood base or box mattress. Insert the firm foam and then the memory foam. Zip it closed. If you don't have a mattress cover, cover the layers of foam with a fitted sheet.