To build a soffit, first gather the necessary materials and tools, including measuring tape, drywall, lumber, a screw gun and a circular saw. Then, decide what size soffit to build, then measure and mark this on the wall. Then, cut the lumber to construct ceiling plates, wall plates and a box frame. Install the plates and box frame to the wall studs with framing nails. Cut the drywall to fit the frame, and attach it with drywall screws.
Check the wall for electrical wires, ducts and pipes, as the soffit should not be nailed into the wall where these features are. Use a miter or table saw to cut the lumber for the plates and box frame. Use either the framing nails or the drywall screws to assemble the box frame with a base, a top and four walls. The posts of the box frame should line up with the wall studs or joists so they can be attached properly.
When fastening the box frame, clamp the frame to the wall so it does not slip or move during the process. Measure where the posts are in the box frame, and use a utility knife to cut the drywall to fit the frame. With the drywall secured to the box frame, finish by sanding, spackling or painting as desired.