Determine how large you want the loft. Measure the loft carefully.
Use the deck screws to build the frame of the loft. The perimeter of the frame will be composed of two 2x4 inches and the cross-members 2x4. The longer section of the loft will be composed of the joint braces. Raise the loft. Screw the 4x4's into the loft.
Place the plywood on top. Screw it into the 4x4s. Put two braces from the 4x4s to the ground to prevent the loft from wobbling. This is done by cutting a diagonal section of the 2x4 and setting it against the 4x4. Set it until it no longer moves. Screw it into the 4x4.
Nail carpet on the top of the plywood. Confirm that the loft is strong. Put the mattress on the top of the plywood.