To begin, you should remove the existing register(s) or grille(s), because proper sizing can only be done using measurements of the actual opening in your wall, ceiling or floor.

Usually, grille sizing is listed as width then height. So if your opening is 22 inches long and 16 inches tall you'll need a new grille or register that is 22x16. Before you install your new grille you can have the interior of the opening painted with a flat black paint, which creates a clean finished look. It may be necessary to have your ducts cleaned, to remove excess dust, before having this done.

The actual measurement of a grille's interior will be somewhat smaller than its size, allowing the frame and damper to fit into the opening, while the flange will be slightly larger. This ensures the new grille will be flush to the surface and completely cover the opening itself. The outer portion of the grille can then be easily attached to the body of the grille, using the built in screw holes.

If you are unable to remove an existing register or grille it is recommended that you get professional assistance, to ensure no damage is done to the duct in the prcoess of removal.