Remove or lay down the third and middle-row seats in your minivan. If your minivan's seats do not lay down flat, removal is a better option. Remove the bolts from the seat tracks and have an assistant help you carry them out of the vehicle.
Measure the cargo area in your minivan, and then measure the mattress. Measure the length and width of each--use this measurement to determine the best position to fit the mattress.
Protect the mattress by putting it in a mattress cover. If you don't have a mattress cover, you can use a tarp or a large sheet of plastic.
Carry the mattress to the minivan. You'll need an assistant to properly carry the mattress. Hold it in a vertical position when carrying it--carrying it horizontally could bend it and cause damage.
Place the mattress into the minivan by inserting through the rear cargo door. The ideal position is flat--just as it would lie on your bed. If you have to angle the mattress, place an item under it for support; be careful not to bend the mattress.