Lay two 2-inch-by-6-inch-by-72-inch pine boards on edge so the outside edges are 36 inches apart. Screw through the end of a 2-inch-by-6-inch-by-39-inch board, into the end of the 72-inch board. Repeat this process on the other end of the 39-inch board and complete the perimeter of the frame by screwing the last 39-inch board to the opposite ends of the 72-inch boards.
Lay one 2-inch-by-6-inch-by-75-inch pine board across the frame, 6 inches from the 72-inch side, and the other 75-inch board 6-inches from the opposite 72-inch side. Screw both boards to the frame using a total of eight screws. These two boards will hold your box spring.
Stand the frame on a 75-inch side. Screw through the inside corner of the frame into a 4-by-4-by-72-inch board so the 4-by-4 is perpendicular to the frame, 1 ½-inches from the 75-inch edge, and the 75-inch boards are between the frame and the ends of the 4-by-4's that will be on the floor. Screw the other 4-by-4's to the 39-inch boards on the frame in the same manner and stand the frame up.
Lay the two 2-by-4-inch-by-6-feet pine boards parallel and flat so the outside edges are 2 feet apart.
Screw the four 2-by-4-by-2-foot boards perpendicularly to the 6-foot boards. Space them evenly.
Lean the ladder onto the frame where you want your child to climb up from and screw in place with four screws.