Watch this video from This Old how to learn how to install an interior door.
1 Close old door and measure up 12 inches from threshold and mark the door.
2 Pull hinge pins and take down old door.
3 Unscrew the lockset from old door.
4 Lay old door on top of new door; keep tops and hinge edges flush.
5 Find 12-inch marks on old door and measure down 11¾ inch; mark new door.
6 Transfer hinge and lockset locations from old door to edge of new door using a combination square.
7 Cut hinge mortises using hammer and chisel.
8 Screw hinges into mortises on door edge; pull pins to separate hinge leaves.
9 Cut door to height using circular saw and straightedge.
10 Screw remaining hinge leaves to mortises in doorjamb, inserting shims if necessary to make hinges flush.
11 Screw hole-boring template to door for drilling out the lockset holes.
12 Bore the doorknob hole through the face of door using a 2½-inch hole saw.
13 Use a 1-inch hole saw to bore latch hole in door edge.
14 Use hammer and chisel to cut a shallow mortise for the latch.
15 Install new lockset.
16 Hang door with hinge pins and check swing and closure.
17 Remove door, prime top and bottom to block out moisture, then paint.
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