ask 10 carpenters this question and you will get 10 answers. here's
mine. take the old door out but leave the hinges attached to the jamb.
fasten the old hinges still attached to the jamb to the new slab
(you'll have to prop it up somehow). (buy a vix bit if you don't have
one.) When you are satisfied that the door will operate, mark the
location of the hinges and mortise out. If you drill in the wrong
place by a small amount, fill the hole with a splinter and try again.
Otherwise, I have used a tape measure working from the top of the door.
But you better have your reading glasses on!