Hanging a door

Need to replace three doors in my late 40's home. One is framed with some odd lumber, it measures 3 by 5 inches. I'm just going to reframe it and put a prehung door in. The other 2 are in frames I can't just remove and replace. Went to a local home improvement center and found doors that are about the right size. They are just blanks. Any tips or can someone point me to some instructions on how to hand these? My thoughts are to remove the existing door, remove all hardware. Clamp the existing door to the replacement and mark the hinge positions. Then try to make a decent cut out for the hinge. Seems to me this is not the best idea but I can't think of a better one.
Al
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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!
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