Preparation
This particular door happens to be a double closet door.
You will need to have a door sized to fit your opening.
It should have 1/4″ or so of gap on all sides of the finish jambs.
Typically the finish jambs are included with the interior door package.
Make sure you order the correct width of finish jambs for the thickness of the wall that you are working on.
You will need the door, 2.5 or 3″ screws that match your hinge color, shims, a cordless drill, door casing trim, a trim nailer, and 2″ trim nails,
First, make sure that the thickness of the wall matches the width of the finish jambs of the door that you will be installing.
Next, use 3″ deck screws to fasten a piece of lumber across the top side of the doorway opposite from the hinges.
Hanging
Set the door into the opening.
Slide two shims into the top hinge side to hold the door in the center of the rough frame opeining.
Now remove the short screw in the center of the bottom hinge.
Insert two shims into the gap between the finish jambs and the framing.
Tighten the long screw until the top of the door has an even gap across the whole width.
Now insert shims in the gap at the other side of the bottom of the door.
If there are no hinges on that side of the door,
you can use a finish nailer to fasten the finish jambs and shims to the framing.
Then fasten the top of the door in place on both sides, using shims and finish nails or hinge screws.
Then fasten the middle of both sides of the door to hold the finish jambs straight and firm.
Finally, you need to score and snap the shims.
If the shims are too close to the hinge pins to access with a utility knife, you can use an oscillating multi tool to cut them off flush with the door.
Finally, cut and nail the casing to both sides of the doorway.
Attach the casing to both the framing and the finish jambs in five places on each side and three places on the top.