Stand the door on edge with the latch-side facing up; check it for plumb. Make a drill guide from wood scraps. Add boards to project the guide from the door and clear the drill. Clamp the guide in place.
Use the lock's template to locate the mortise holes. Mark the holes' centers with a nail. Drill the holes with an auger bit. Chisel out between the holes to square off the mortise. Then chisel a shallow recess for the lock plate.
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Tape measure and folding ruler
Drill/driver with auger bits
Measure the width of your opening, jamb to jamb, in three places (top, middle,bottom) and the height, threshold to jamb, in three places. Note the largest of each and use these as a guide to buy the door.
2. KNOB SET
A set for a cylinder lock should include two knobs, a spindle, and two round escutcheon plates. For a mortise lock with a bolt, two escutcheons or a long backplate with a keyhole may be required.
Choose between a cylinder lock (a tube-shaped ;acthset with a lock on the knob) or a mortise lock (a rectangular set with a bolt and a skelton key). You can reuse an old lock, but a new mortise lockset might be worth the cost (less than $20) just for the mortise-cutting template that comes with it. Before buying a mortise lock, make sure the distance from the spindle hole to the keyhole matches the one on the backplate you choose, and that the lock body isn't too thick to fit your door.
You will likely need three to hanadle the weight of a solid-wood door or solid-core door. Many doors come with instructions that indicate the size and number needed.
5. LOCK INSTALLATION KIT
If necessary, for drilling for a cylinder lock. Tool companies make these kits, which come with two different hole saws that attach to a drill, along with a jig to guide where the holes go. Some kits also include a drill-mounted router bit and jig for carving out shallow mortises for the lock's faceplate and the strike plate.
6. WOOD SHIMS
To hold the door in its opening as you check the fit and mark for hinges.