First off there’s quite a lot of tools you’ll need I’ll introduce them a long the way as needed. You’re going to want to measure your new door and make sure it’s the right size by measuring the height, width & jamb once you have confirmed that by double checking you’ll want to proceed with the removal of the existing door. You will need a razor blade to score around the trim so that the paint on the wall can remain in tact when you remove the interior trim. To remove the trim you’ll need a hammer and a crow-bar you want to start on the inside reveal (so that you don’t damage the wall) once you’ve removed the interior trim take the outside trim off as well. Now open the door and remove the screws from the hinges with a drill gun then once the slab is off it’s time to remove the frame and threshold with the hammer and crow-bar. Then remove hit the nails in that are in the jamb that were left from the old frame.
Now it’s time to sweep the the dirt and debris with a broom out of the opening and apply a bead of silicone from jamb to jamb to prevent water from getting in underneath the new threshold. Place the new door into the opening and snug it up with shims plumb, level, and square and make it flush as much as possible to the inside wall to prevent a space in between the interior trim and the frame. The shims go in on each side of the frame top, middle, and bottom. Make sure the door opens and still shuts constantly as you secure it with screws. You might need to compensate with walls that are out of plumb to give your door’s trim it’s best potential appearance for the situation. You must remember no house is built perfect, which is definitely the case 99% of the time. Now that the door is in place put screws through the shims to lock it down completely, again after each screw continue to make sure the door still opens and closes.
Onto the last phases which are insulating, (insulation is optional) trim, & sealing the door. The angles you choose to cut into the trim should match the other doors on your house. To figure out the width for the trim it’s good to measure 3/8 of an inch (if your frame is 3/4 of an inch thick, adjust accordingly) to the inside the frame to the existing paint-line. To measure the length of the legs for the trim measure from the ground/floor to the inside top of the frame then add an 1/8 of an inch for a reveal. Now shoot those legs on preferably with a compressor & nail gun shoot roughly every 6 inches 1/8 of an inch from the inside of the frame. (same as the top piece of trim’s reveal) Repeat these steps for the outside trim the only difference for the outside trim is that you want to measure to the bottom of the frame instead of to the ground. Last but not least you’re going to need a case of paint-able silicone, a bucket of water, a sponge, and a caulking gun, to keep water from getting behind your trim and damaging it. Give the silicone a few days to dry and it’s good to paint, and you finally got rid of that old door!