First steps would be to decide what materials. People usually go with melamine, PVC sheeting or plywood. PVC is light and water resistant, but can be hard to come by in regular hardware stores, and can get pricy. Melamine is basically compressed particle board with a plastic coating on it making it water resistant, but is extremely heavy. Any plywood will work well, but keep in mind it must be sealed to prevent water damage.

The internet has many basic plans available. Essentially you are just building a book case, that you will wire heat to. Reptile basics and Pro Products can sell you pre-wired heat tape that all you need to do is tape down. Put on a quality thermostat and youre good to go.