It is often best to lightly sand the boards if they need it prior to construction as it can be difficult to get into the crevices.
Build the platform storage base. You will build two of these to make the bed frame base. Build the outside frame and then add the inner slats evenly spaced apart. Join all pieces with wood glue and nail from the outside into the slats. Pre-drill prior to nailing to ensure plywood does not split. Nail or screw the backer board on.
Build the headboard. The center plywood panel sits flush with the back of the 2x4 headboard. Cut the headboard first and then pocket screw the plywood to the frame according to the diagram. If one is particularly hard on furniture, considering adding a cross brace between the legs and attaching to the platform with extra screws.
Build the footboard. The footboard is put together similarly to the headboard.
Secure the headboard and footboard to the base. Place the platform/cubbies in desired location leaving the appropriate spacing. Place the headboard and footboards in place at either end and lag or screw headboard and footboard in place according to diagram.
Finishing. If desired, finish the exposed plywood edges, glue on thin hardwood banding for a stained bed or wood fill if your plan is to paint. Fill any holes or knots if necessary. Paint or stain as desired.