Remove the side rails from the double headboard. Depending on the style of bed you either need to unscrew the bolts with a wrench or slide the brackets out of place. Discard the side rails and any footboard or store them to convert the headboard back to a full size at a later date.
Attach a queen converter bracket to the headboard on either side. Converter brackets are available at mattress and furniture stores. You can choose from bolt on converters or converters that have brackets that slip into the existing slots. Choose the converter that matches the way the side rails were attached. Follow the manufacturer's instructions on the proper attachment of the converters. Use the included hardware for proper installation.
Attach a metal queen bedframe to the attached converters using bolts. Insert the bolts through the headboard and into the bedframe. Secure the bolts using nuts.
Set your bedspring and mattress as you normally would.