Remove the door from its hinges by placing a screwdriver under the top lip of the hinge pin and tapping with a hammer to drive the pin out.
Remove the threshold and doorjamb. If the jamb proves difficult, use the reciprocating saw and the metal-cutting blade to cut through the nails, then pry the doorjamb off. Replace any damaged or rotting wood, as it will be against your new doorjamb (this is a situation where you do not know what the situation will be until the old door and jamb are removed.)
Remove any remaining caulking and lose debris around the doorway, then clean the work area. Measure the opening and the door to check the fit. Use 2 x 4s to frame the door and fill the area between the door opening and door size if needed.
Use the long level and carpenter's square to make sure the inside of the opening is square and true.
Bring the door up to the opening for installation, sliding the bottom of the door in first. To check square, measure diagonally from corner to corner of the frame. Also check for plumb with a level.
Install the new frame and door assembly, using shims to level the door and make it close and open properly. Drill pilot holes through the frame and shims where the door hinges are. Drive nails just far enough to temporarily hold frame in place.
Secure frame permanently with 3-inch deck screws through the rest of the holes in the hinge assemblies.
Trim the shims so that they are flush with the wall, using a utility knife to score them first.
Measure opening for new trim details and cut trim. Attach trim to door opening using a small pneumatic nailer.
Attach detailed/finish trim as desired.
Use a silicone caulk to seal all the joints and paint as desired, then install door hardware according to manufacturer's instructions. Repeat steps 5-6 for the exterior side of the doors.