Remove the existing door if there is one by popping the pins out of the hinges. Tap a finish nail into the hole in the bottom of the hinge barrel. This forces the pin out the top of the hinge. Separate the hinges and take the door out of the frame.
Place the Masonite solid core door in the door frame where the old door was. Fit the hinge halves that are attached to the door into the hinge halves that are attached to the door frame.
Put the pins back into the hinges and tap them down so they engage both hinge halves and hold them together.
Open and close the Masonite solid core door several times to be sure that it is functioning properly.