Clear the area around the doorway and cover any furniture in the room with plastic covers.
Measure the width of the doorway and trace an arch on a piece of cardboard using a pencil. The curve should allow enough room underneath the doorway for you to pass through.
Remove the drywall around the doorway with a utility knife to expose the framing.
Extract the corner beads from the corners of the drywall using a hacksaw. Measure from the floor to the highest point of the arch and cut the drywall to that height. You should be left with a rectangular cutout in the doorway.
Place the cardboard outline of the arch over two pieces of 1/2-inch plywood and cut them out.
Nail the plywood arches to the door frame using 2-inch nails, ensuring both the arches are flush.
Measure the underside of the arch and cut a piece of drywall to that length. Soak this drywall in water for 30 minutes or until it bends easily.
Position the drywall at the bottom of the arch and push it against the entire underside of the archway, securing with 2-inch screws every six inches.
Cut drywall to coincide with the measurement of the archway and screw it into place on both sides of the arch using 2-inch screws.
Mix joint compound and apply two layers to the entire archway.
Sand the arch and let it dry. Paint the arch according to your desire.