Chiselling a recess---called a mortise---for a door lock plate is one of those jobs that's very simple in theory but also very easy to mess up. If things don't go well for you the first time, your lock will probably work fine after a few adjustments, but you'll likely have to view the evidence of sloppy work for the life of the door. So, take the time to mark and cut carefully. Whether you're installing a lock plate (a.k.a. strike plate) for a deadbolt lock or a standard lockset, you first must install the lock or lockset onto the door, following the manufacturer's instructions.
Close the door, and look for where the lock bolt or latch bolt contacts the door jamb. Mark the precise centre (top to bottom) of the bolt onto the inside edge of the jamb using a pencil. Use a combination square to transfer this centre mark across the face of the jamb (onto the flat strip between the inside edge of the jamb and the door stop).
Measure and mark a line along the centre (side to side) of the jamb strip, intersecting the line made in the preceding step. At the line intersection, use a spade bit to drill a hole for the bolt as directed by the lock manufacturer. Note: For flat-sided deadbolts, you typically drill two overlapping 7/8-inch holes centred about 5/16 inch above and below the horizontal centre line.
Position the strike plate on the jamb so it is centred on both lines. Score around the plate with a sharp utility knife. Using a sharp 1-inch-wide chisel, cut a series of parallel lines into the jamb about 1/4 inch apart, staying within the scored lines and holding the chisel perpendicular to the wood.
Remove the waste wood with the chisel held nearly parallel to the jamb. Clean out the mortise until its bottom is smooth and flat and at a depth exactly equal to the thickness of the strike plate. Test-fit the plate as you work to check the mortise depth.
Drill pilot holes for the strike plate, and fasten it to the jamb with the screws provided.
Here's an easy way to mark the side-to-side position of a strike plate for a standard lockset: Draw along the vertical edge of the latch bolt (adjacent to the flat side of the bolt) with a pencil. Turn the knob, and hold it to keep the bolt retracted. Close the door tightly, and let go of the knob. The bolt will make a pencil mark on the door jamb. Align the edge of the strike plate's hole just to the door-side of the mark, and proceed with scoring around the strike plate.