Remove the drywall using a utility knife until the framing of the doorway is exposed.
Cut the 2inch-by-3-inch lumber into three equal pieces to go along the sides and top of the doorway frame. Screw these pieces into the framing using 3-inch screws.
Draw the arch on a piece of plywood with a pencil, making sure the dimensions coincide with the framing.
Stack the plywood pieces together and cut along the drawn circumference using a handheld router so that both plywood arches are flush.
Position the plywood pieces on either side of the arch frame and secure them into place using drywall screws.
Attach the wooden blocks every two inches along the inner radius of the arch using drywall screws.
Soak a piece of drywall in water for 30 minutes to an hour so that it bends easily.
Line the underside of the arch with the drywall piece and fasten it into the wooden blocks using drywall screws.
Apply drywall to the entire doorway arch and cover with a double layer of joint compound.
Sand and paint the arch.