Remove the mattress and box springs from the bed, leaving the bed frame exposed. You'll need full access to the end of the rails, so clear away any other nearby furniture so you can easily move around the end of the bed.
Set a drop cloth under the end of the assembled Crate & Barrel queen bed frame. The cloth will catch the bits of metal shavings that come from drilling into metal. Crate & Barrel queen metal bed frames do not come with pre-drilled holes on that end of the bed.
Pry the end caps off the rails with a screwdriver. Hold one footboard adapter up to the end of the rail. Mark where the footboard adapter bolt holes will go at the end of the bed frame with a pencil. Repeat on the other rail.
Drill through the metal bed frame at the places you marked. Hold the drill so that the bit is parallel to the floor for the straightest holes. Wipe away any metal shavings when you finish.
Hold one footboard adapter up to one bed rail. Screw in the bolts that came with the adapter kit and fasten them with the supplied nuts. Use a wrench to tighten the nuts. Repeat on the other rail.
Align the footboard bolt holes with the bolt holes on the footboard adapter plate. Insert a bolt on one side and secure it with a nut. Tighten it with a wrench. Repeat with the other side of the headboard.
Move the bed into place and replace the box springs and mattress.