Remove the mattress and box springs from the metal bed frame. You'll need clear access to the ends of each bed rail.
Position the footboard bracket against the end of one metal rail. Align the screwholes on the bracket and on the rail until they match.
Insert the screws that came with the footboard kit. Tighten them until the footboard bracket is securely attached. Try to wiggle the bracket to ensure that it is not loose. A loose footboard bracket will result in a wobbly bed.
Repeat Step 4 on the second footboard bracket.
Hold the footboard in place at the end of the bed rail with the new footboard brackets in place. Align the holes on the footboard with the holes on the brackets. Insert screws in the holes, and attach the footboard to the bed frame. Put a screw in one side, then a screw in the other side, so you don't have to hold the footboard for long. Alternate between the sides of the bed as you insert screws to make sure that the spacing of the footboard stays consistent.
Replace the box springs and mattress, and remake the bed.