Remove the door from the frame
Mark the hinge locations on the door edge
Secure the door on a stable work surface
Use a utility knife to score around the hinge outline
Use a chisel to remove the hinge mortise material
Cut along the marked lines with a sharp chisel or router
Remove small amounts of wood at a time
Test-fit the hinge frequently
Adjust the mortise depth until the hinge sits flush
Drill pilot holes for the hinge screws
Reattach the hinges to the door
Rehang the door and check alignment