Remove the bolt. With a chisel, cut a mortise for the plate. The depth of cut should be the thickness of the plate. See Step 4, above. Screw the parts together from the interior.
Some deadlocks have a double strike plate.
That is, the flush-mounted plate is backed up with a reinforcing plate. If your lock has this feature, drill the hole for the bolt and mortise for the flush-mounted plate. Install the reinforcing plate first, using 3 in. Then screw on the flush-mounted plate. If the bolt has to be extended to go into the deeper bolt hole, adjust the cylinder and thumbscrew along the bolt where the cylinder assembly goes through.