Place two hollow doors together with wood glue, or cut a large piece of 2-inch-thick wood for your platform base. It must measure at least 75-inches long and 39-inches wide for a twin bed, and up to 78-inches long and 80-inches wide or more for a king-size bed, depending on how large your teen wants the platform to be.
Nail a 2-inch by 4-inch post underneath the platform, down the middle and along the seam where the doors meet, if you're using them. Center the board vertically. It should measure the length of the doors minus 8-inches for additional boards that will line the platform's perimeter.
Nail four more 2-inch by 4-inch boards underneath the platform along the perimeter. Trim them if necessary so they are flush with the door edges.
Attach furniture legs between 6.5 and 10-inches to each corner of the platform bed, just inside the 2-inch by 4-inch boards. Follow manufacturer instructions for complete installation. Install three more legs down the length of the center board. These legs should measure 2-inches shorter than the others, since they will attach directly to the center 2-inch by 4-inch board.
Have your teen choose a paint color for the bed.
Prime the finished platform, if you're painting it. Let it dry completely.
Paint the platform your teen's desired color. Let the paint dry.