Remove packing braces and nails from the interior door. Set the closed door in the rough opening, and center it. If the floor is uneven, you may need to carefully trim the appropriate edge from the lower doorjamb before you can install the door.
Use a level to vertically align the door. Guide with the hinged side of the door.
Put shims in place to make the door plumb. The shims go between the doorjamb and rough framing. Tap them in from both directions.
Nail through the doorjamb and the shims into the wall stud. Start at the top hinge. Use 8-penny finishing nails. Make sure all nails go through the shims behind the jamb.
Repeat this process for all the other hinges. Check the door's alignment, then put shims and nails in the rest of the door. When you finish, the space between the door and the jamb should be equal all the way around.
Trim the shims carefully. Don't cut the jamb or wall.
Cut the door's trim. Miter the corners. Nail the trim every 16 inches. Use 4-penny finishing nails. Carefully set the nail heads without damaging the trim.
Finish the door. Install the doorknob. Most prehung doors come with the doorknob hole already cut. Finally, install the latch.