Existing Hinges
If you intend to use the existing hinge recesses in the frame, the positions need to be marked on the door.
Position and support the door in the opening at an angle to the upright and mark on it where the tops and bottoms
of the hinge recesses are.

New Hinges
The positions need to be marked on both the frame and the door.
Interior doors should be hinged about 125-150mm or 5 to 6 inches from the top and 175-230mm or 7 to 9 inches from the bottom of the door.
Use either brass or steel butt hinges either 75mm (3 inches) or 90mm (3½ inches).

Chisel out the recess carefully and fit the hinges to the door. Make pilot holes first for the screws to avoid splitting the wood. Make sure they fit flush and lie neatly in position.

You will need to support the door in position in the doorway and fix the hinges to the frame, use one screw in each hinge. Before completing the job, check that the door opens and closes correctly. If there are problems, remove the two screws and correct by making minor adjustments where necessary to where the hinge sits in the frame.

Repeat the procedure, test open and closing and secure the hinges completely. Decide on the lock or latch most suitable for your door, perhaps with locking in mind or a match to the other internal doors.