Building scenery for the stage is half carpentry, half illusion. Standard carpentry techniques are used to construct practical doors, which are those that can be opened and closed. The surrounding wall section, or flat, is typically built attached so that the door becomes a modular wall unit that can be reused. Start with a hollow core prehung to shortcut the process. The attached wall can be built from 1/4 inch plywood or masonite.
Lay a sheet of plywood or masonite on a pair of sawhorses or other level, sturdy work surface with the face that is set farthest back in the jamb down to give the door the most depth possible.
Measure the door jamb from outside to outside. Cut a piece of 1-by-4 to that measurement with a circular or miter saw. Remove any bracing attached to the bottom of the jambs. Attach the 1-by-4 to the bottom of the jambs with 1-5/8-inch drywall screws. This will hold the jambs in position and provide balance to the flat.
Position the door centered on the plywood with the bottom edge of the new 1-by-4 lined up with the bottom edge of the plywood. Mark around the outside of the jamb up both sides and across the top to make a rectangle the size of the outside of the jamb.
Stand the door frame up. Cut a 1-by-4 to fit the two side jambs. Cut them to length so the top of the 1-by-4 is even with the bottom edge of the horizontal jamb header. Nail these to the face of the jambs. Measure from outside to outside of the 1-by-4s. Cut a third piece to that measurement. Nail it to the horizontal jamb header so the outside ends match up with the outside edges of the 1-by-4s on the jambs.
Cut out the rectangle in the plywood with a circular saw so that the sheet now has a U shape. Fit this U shape around the door jamb from the back so that the plywood rests against the 1-by-4 trim you added to the face in the last step. Staple the plywood to the back of the 1-by-4.
Staple one 8-foot 1-by-4 to the back of each long edge of the plywood. Place the 1-by-4s with their wide faces against the plywood and the edges lined up with the outside of the plywood. Cut a piece of 1-by-4 to fit along the top edge of the plywood between the two 8-foot 1-by-4s. Staple it to the back of the plywood.
Measure the distance between the edge 1-by-4s and the door jamb at the bottom on either side of the door. Cut a piece of 1-by-4 to fit each side and staple it to the back of the plywood. Cut two 6-inch squares from 1/4-inch plywood. Cut those in half diagonally on a miter saw at 45 degrees. Line them up with the outside edges and staple them to the backs of the 1-by-4s at the corners of the flat for reinforcement.
Your door will be hung to open to the offstage side. Choose a right or left hinged door according to the needs of the set.