You need to create a 2x4 frame that is the right dimensions to support the plywood platform that was made in the previous step.
The height of the box will be the height that you just decided.
In order to properly assemble the Entire 2x4 frame, you need to first make 2 rectangular end caps (the sides of the bed). These should be the height that you decided and the width of one of your plywood sheets from the bed frame. In my case, those rectangular frames were 18” tall and 36” wide. Assemble these frames.
Next, you will use 3 2x4s that are cut to the width that you want your bed to be. Mine was 55". Connect one 55" 2x4 to the lowest point on the front of the frame. Then connect the second to the lowest point on the back of the frame (against the wall of the van).
Add another 2x4 of equal length to the rear of the frame (against the wall of the van) and attach it securely to the wall of the van.
The next 2x4 should be the same size as the previous 2 and will be attached to the upper part of the front of the frame. Make sure that this piece is not attached flush to the top of your frame. Allow at least 3 inches of clearance from the top of the frame to ensure that the drawer slide is able to move freely once it is attached in the next step. At this point, the 2x4 frame should be stable, but it is recommended to add extra support where needed.
Now its time to attach the drawer slide to box frame. It is important that the front support piece is lower than the sides to allow clearance for the drawer slide (noted in the last step).
Regardless of the type of drawer slide that you choose to use, it will most likely come in two parts: a stationary slide, and a movable slide that fits inside of it. You'll need to attach the stationary pieces to the inside of each side of the frame.
The movable piece of the drawer slide needs to be connected to a separate piece of wood that will be used as an anchor for the sliding bed. In my case, I used some 2x2s that I already had lying around.
Once the 2x2s are connected to the movable slide, unite the two parts of the drawer slide together. With most slides there will be a locking mechanism to keep the two pieces together once connected.