Here is how I do that.

Take off the old door and remove the hardware.

Lay the old door on top of the new door---being careful to orient the new door top to the old door top. Make sure the thickness is the same.

Mark any differences in length or width and trim to size.

Using a square and a metal ruler, make the marks for the hinges. If the new door is thicker/thinner than the old, you will have to move the hinge mortises in or out on the new door to match the door stop on the old frame. Basically on a thinner door, you will need to mark the mortise vertical line closer to the outside of the door---which takes more of the edge for the mortise. A thicker door mortise will be less of the edge.

Cut the mortises, drill at least one hinge screw hole and mount the door to the frame with one screw in each hinge. Check the fit and make any adjustments necessary. Then install all the screws in all the hinges.

Buy a door hardware jig---$20 at home improvement stores---it has the locater jig and both hole saws necessary to correctly drill the two holes. If you are using the old hardware, install the jig on the old door----to find out which part of the opener hole in the jig to use. This locates the hole for the correct setback of the latch---the distance from the edge to the center of the hole. There are two common setbacks and the jig is designed for both.

Mark the top and bottom of the opening of the striker plate on the door edge and position the jig so the edge locater hole is centered between those lines.

Drill the large hole and then the smaller hole.

Install the latch hardware and test.

I am going to be installing a Mahogany Door this weekend, with the casing as well. I have a 1950 home with a good amount of dental tooth trim around the outside on the exterior part. What would be the best way to go about this instalation? I have also never installed a full door before. Any help would be appreciated. Thanks.