First, the jamb opening should be as square, plumb, and level as you can make it; otherwise, you’ll need to make adjustments to the door. Choose a door that’s the same size or slightly larger, as free of warping as possible. We strip old paint and repair loose joints at the beginning, then reglue, apply epoxy, fill in old hinge mortises, and do a sanding.
We chose antique, cast-iron hinges, a little tricky to work with. To mark locations, one of us held hinges in place while the other made pencil marks on the jamb. We cut mortises using a standard mortising template and a router with collared bit. Setting the correct depth on the router is key. Alex used a plunge base with the setting zeroed out, the bit flush with the jamb. He pulled the stop on the plunge base up slightly and used one side of the hinge to set a gap between the router stop and the base. After removing the hinge, he let the bit plunge all the way down to the stop to reach the perfect depth. (Save settings for mortises on the door.) After chiseling out excess material in the mortise, we mounted jamb side of hinges into the door frame.
The door should be a little tight in the opening. Using a small laser measure, we determined height and width of the jamb opening. We measured at three places across top of jamb, and three down the sides. If it is seriously out of square, make a template of the opening (use thin MDF); use it to transfer measurements to the door.
To trim the door, start at the top, then remove a similar amount from the bottom. We marked a cut line, then used a track saw to remove excess. Before trimming the sides, we measured the total amount to be removed, then split the difference between the two sides before first cut.
To mark hinge locations on the door, place door in the opening and force it to the top with shims. Pencil locations of hinge mortises on the jamb, top and bottom. Transfer marks onto side of the door with a carpenter’s or rafter square. Tack hinge template to side of door. With router set as before, cut mortises; chisel any excess material and mount hinges. Then test-fit the door.
The hinges may add width. If so, close the door as far as possible and mark a new cut line top and bottom, based on where the edge of the jamb sits. Cuts can also minimize a minor out-of-square in the jamb. Pop the door back on the hinges.