Determine whether you need to make an L-shaped soffit or a U-shaped. L-shaped soffits are used when the ductwork sits against the wall.
Take the measurements of ductwork. Include approximately 1/2inch extra in the measurements for each side of the heat duct. The duct work should have small space around all sides of it once the soffit is constructed.
Use a miter saw to cut a 2x3 stud for the soffit using the ductwork length measurements. U-shaped soffits need two studs.
Space the studs ½ inch from the ductwork. Screw the 2x3 support stud into the ceiling joists.
Build a number of L-shaped (or U-shaped if the soffit is not against a wall) pieces for the soffit frame. Cut the pieces for the framing on the miter saw. Screw the pieces together with a screwdriver. The L-shaped frames should slide over the ductwork.
Space the frames so they are located every 16 inches on center. Screw the frames into the studs around the heat duct. Use a level to ensure that each framing piece is level and plumb before screwing it into place.
Cut pieces of sheetrock to fit over the soffit frame. Score the dry wall with a utility knife and snap it from the other side. Use the utility knife to finish cutting the sheetrock paper.
Attach the sheetrock to the soffit framing with a screwdriver and drywall screws. Cover the seams in the sheetrock with drywall compound and drywall paper tape. Let the compound dry overnight. Sand the compound smooth. Paint the soffit with latex paint.