Clear an area on the floor about the size of your mattress. For a twin bed, this will be about 3 feet wide by 6 feet long.
Set the rails on the floor, running parallel to each other, about where the sides of the bed will be once assembled.
Unfold the top and bottom rails from the side rails. This often works like opening a pocketknife.
Overlap the top rails from both side rails until the pre-drilled holes overlap. Connect them using the nut and bolt that came with the kit.
Repeat Step 4 for the bottom rails.
Set the box springs inside the frame formed by the rails. Add the mattress to the top.
Set the headboard and foot board on the ground, about where they will be once the bed is assembled. Lean the headboard against the wall.
Set the side rails in position between the head and foot boards. Position them so the raw wood and rails face inside, toward each other.
Attach the side rails to the headboard. The specific method will vary from design to design. Two common methods use a bolt or screw passing through the headboard or an anchor plate inside the headboard.
Repeat Step 3 to attach the side rails to the foot board. In many cases, this will be the same method as with the headboard.
Slide the box spring and the mattress into the frame formed by the boards and side rails.