Place two milk crates side by side with the open ends facing the floor. Connect the two crates by zip-tying the sides together at three points.
Add two more crates in the row by zip-tying them together in the same manner. This creates a row of four crates.
Make four more rows of four crates, for a total of five rows. Connect all the rows together to create a solid rectangle of 20 milk crates.
Ask a lumber yard or home center to cut a plywood sheet to measure 54 by 75 inches. This is the size of most American double bed mattresses.
Sand the edges of the plywood sheet until they're smooth, to prevent splinters.
Spray the entire surface of the plywood sheet with a light coating of white spray paint. Allow the paint to dry thoroughly, then add another coat. Let the paint dry overnight to make sure it's completely dry.
Place the block of milk crates in your bedroom where you want your bed to reside. Lay the plywood sheet on top of the milk crates, centering the plywood on the milk crate base. Lay the mattress on top of the plywood and make your bed.