exterior door installation
When you buy a door, it will probably be a pre-hung frame. Pre-hung doors usually come with wood or steel frames. You will need to remove an existing door frame from the rough opening before you install a pre-hung door. The door frame must be as square as possible, so that the door seals tightly to the jamb and swings properly.
Before adding the interior trim, apply an expanding foam caulking to seal the new door frame to the rough opening and threshold. This will help prevent air from getting around the door seals and into the house. Apply carefully, especially with a wood frame, to avoid having the foam force the frame out of square.
If needed, you'll also want to add weatherstripping. Check the weatherstripping on your exterior doors annually to see if it needs replacement.
See also exterior door selection.