I use a Mortise Jig for installing mortise locksets, since I install alot of commercial mortise locksets...but you can accomplish this without one.
#1. place the mortise template on the edge of the door and trace the area on the door to be mortised. using a sharp 3/4" or 1" chisel (whatever the width of the mortise is) proceede to chisel out about 1/8" deep the entire sqaure of the area of material to be mortised. #2. Using spade bits (otherwise known as paddle bits) start "progression" drilling the material from the area. do this by drilling down 1/2 inch , them moving the bit down 1/2 inch to the next spot with higher material, until you've reached the level you need to insert the box. I usually have a piece of tape on the bit the depth of the box.
use the chisel to remove the excess material from the edges of where the bits did not get. Once you get to where you can place the box into the cavity, insert the box all the way down and trace the the face plate. then chisel out the depth of the face plate where you just traced into the edge of the door.
take your time and think everything though. otherwise most reputable door companies will do this for you for a fee.