Set the metal bed frame on a flat surface. Attach any crossbeams, making the bed sit as it would with a mattress and box spring on it. Measure the width of the headboard, footboard and bed frame. Adjust the bed frame, making it fit the width of the headboard and footboard.
Lean the headboard against the top of the bed frame and adjust the headboard until it sits flush with the frame. Locate the thin slots in the frame and mark where the slots fit against the headboard. Repeat the process with the footboard, if the bed has one.
Predrill several holes, following the marks you made earlier. Make the holes approximately 1/2-inch to 1 inch, depending on the size of your screws. Create two holes per slot on the bed frame, for a total of six to eight holes. You should have a hole on each side of the slots, which keeps the frame from moving.
Slide a plastic anchor into each drill hole. The anchors add a layer of protection between the metal screws and the wood on the bed and keep the screws from splintering the wood. Move the headboard against a wall, which keeps it from tipping over. Transport the frame to the same spot and line up the holes.
Press the frame against the bed and keep one hand on the frame, stopping it from moving around. Place a wood screw through the hole on the frame and into the bed. Carefully screw each one into place. Repeat the process on the footboard of the bed.