When door frames are uneven, it can be challenging to case the frame. A simple 45 degree angle leaves a large unsightly gap. Uneven door frames are quite common in older homes and are usually caused by settling of the building. If the building is still moving, repair that problem first or large gaps will appear in the new casing and could cause the door to bind against the jamb.
Cut a 45 degree angle on two small pieces of casing about 10 inches in length.
Hold the two pieces up in the left corner to see where the gap is.
Adjust your mitre box and cut both pieces of casing so that the mitre is tight and both heals and points meet without overlapping. This may take you a couple of attempts to get the right angle. If the angles are too different from one another, you will create a mismatch where the points do not line up.
Cut the angle on the left piece of casing, cut it to length and nail it to the door frame and wall. Use finish nails that go into the jamb at least 3/4 inch and at least 1 inch into the studs. Hold up a scrap piece of casing against the head jamb and measure up to the top of the casing to determine the length. Leave it a little long for the first cut just in case you need to adjust the bottom.
Cut the angle you determined on the left side of the head piece leaving it 3/4 inch long and a 45 degree angle on the other end.
Hold up a scrap with a 45 degree angle to the right side of the head piece and adjust the angles on the head and scrap piece until you have a good-fitting mitre. When the angle is determined, cut the head piece to length.
Apply glue to the left mitre of the head piece and nail it to the frame and door with finish nails.
With your mitre box still set to the angle you determined for the right side of the head casing, cut the mitre on the right piece of casing, cut it to length, glue the mitre and nail it to the jamb and wall.
Casing can be left off the floor about 3/8 inch if carpeting will be installed; otherwise, cut it flush with the finish floor. If your jamb is set in from the wall or sticks past the wall, you will have to tilt the mitre box to make the casing meet on the face. This is called a compound cut.