Slanted doorways can be tricky when it comes to installing doors. It might not seem obvious at first, but when you hang the door and shut it, it sticks or binds on the doorjamb. If you try to adjust the jamb, you have to remove the casing, pry the jamb from the wall and typically wind up with worse problems. The most efficient way to deal with crooked doorjambs is by fitting the door to the jamb.
Screw the hinges to the doorjamb, using a drill/driver and 3/4-inch screws. Stand the hinge side of the door inside the jamb. Open the door at a 30-degree angle as if it were already installed on the jamb. Use wooden wedges at the bottom. Ask an assistant to help you steady the door.
Swing the hinges over and put them in place on the side of the door as if they were already installed on the door. Trace around the hinges with a pencil. Remove the door and place it horizontally on its edge with the hinge side facing upward. Clamp it to a sawhorse to steady it.
Place the tip of a chisel vertically onto the tracing. Tap the chisel with a hammer to cut around the perimeter of the tracing to a depth of 1/8 inch. Place the tip of the chisel, beveled side up, horizontally at the opening of the tracing. Tap the chisel forward to cut the tracing out to a depth of 1/8 inch. Do all the tracings.
Remove the doorknob -- if the door has one -- using a drill/driver. Hang the door on the jamb, using 3/4-inch screws, the drill/driver and the hinges on the jamb. The hinges fit into the cutout tracings
Shut the door as far as it will go without sticking in the doorjamb. Trace down the side of the door from top to bottom, using the side of the jamb as a guide. The pencil may not make contact with the door at some point as the line tapers indicating the slant. Unscrew the door from the hinges.
Place the door flat across two sawhorses with the slanted line facing upward. Clamp it to the sawhorses at both ends to steady it.
Install an 80-grit belt on a belt sander. Hold the sander with both hands horizontally, parallel with the side of the door. Begin sanding the edge of the door to the line. If you only have a line on part of the door, then sand only that part of the door. Remove the line completely, using the sander.
Hang the door back on the hinges. Repeat Step 5 as many times as needed until the door shuts normally without sticking or binding in the doorjamb. Once or twice is all you should need.
Sand and round the corners and edges of the door by hand, using 100-grit sandpaper. Apply stain as needed. Spray the edge of the door with two coats of aerosol lacquer. Install the doorknob.
Shut the door. If the striker on the door fails to penetrate into the hole in the jamb, remove two screws holding the striker plate to the jamb, using the drill/driver. Move the striker plate -- the small brass plate on the side of the jamb with the hole in it -- up or down until the striker on the doorknob fits cleanly into the striker plate.