1. Go measure your cab
2. Go online and research the recommended air volume for each sub
3. Decide if you want a sealed or ported enclosure
4. Either do some math and figure out how to get that much air space behind the seat, OR go online and use a free sub box calculator and try dimensions that will fit until you achieve the recommended air volume.
5. Now either look online to purchase a box that will fit, or collect the materials it will take to fabricate one.