Mark and drill mounting holes on the bed frame, approximately 1-inch from the top and bottom. Drill the holes on the head of the frame box or, if your bed has legs, on each leg at the head of the frame. If your bed has a box frame, slide a carriage bolt through the hole you drilled and secure it from the inside with a lock washer and hex nut. If the bed has legs, screw a carriage bolt into each hole until tight.
Position the headboard and mark it for mounting holes. If the headboard is full size, you need to trim the legs to fit. Measure and trim as necessary. Drill all the way through the headboard leg.
Connect the headboard to the platform bed frame. Slide the headboard holes through the carriage bolts, placing it against the bed frame. Slide a lock washer through the carriage bolt and screw on a hex nut. Tighten with a wrench. Repeat the process for the remaining connection.