Write an article based on this "Gather your tools. Select the new door. Check that the new door will fit. Remove interior and exterior trim. Create rough opening around frame. Make sure everything is level."
There are a number of tools you're going to need to remove the old door and put in the new door. Make sure that you have what you need before you start trying to do this operation. If you don't have these in your house, a trip to the nearest hardware store is in order.   Leveler to make sure that everything stays level (you'll have difficulty opening your new door if it's at an angle). Caulk to fix the insulation and framing in place. Hammer and nails, nail set, screwdriver (depends on the nails holding your old door in place) to remove the old door and put on the new door. Tape measure or ruler to measure the old and new doors and frames. Insulation to make it so those winter winds don't start whipping around your door. Wood shims to hold the door at the right level, if needed. Before you start ripping your old door out of its frame you want to get the new door. The size and type will depend on your needs and the size of the door frame. You don't want to get stuck with the wrong-sized door.  Wood doors tend to have a nicer look, but they aren't as weather-resistant as fiberglass or steel doors. Steel doors also tend to be the least expensive, while wood and fiberglass tend to be around the same price depending on the style of the door. There's nothing worse than getting all primed to go with your new door only to discover that it is too big or too small. You can avoid this unfortunate situation by taking measurements of the old door and applying those measurements to the new door.  To check the width you'll need to measure across the top middle and bottom from stud to stud of the old door. The stud is basically the side of the door. The smallest number is the rough estimate for the width. For door height you're going to need to measure the middle, and both sides of the door from the top of the door to the floor. The smallest number is again going to be your rough estimate. Measure the width of the doorjamb. Check the measurements of the old door against the measurements of the new door you're considering. If they match up closely, then you're good to go. If not, you're going to need to consider a new door. This requires removing the door as well as the existing exterior trim and old insulation. Before you can remove the door and main frame, you must cut the caulking, remove the interior and exterior trim, and remove the 1×4 nailing boards around the exterior of the frame and any shims or nails sticking out of the trimmer studs. To help minimize dust during the demolition, remove the door towards the end of the process prior to extracting the main frame. Using your hammer and a nail set (or a screwdriver), remove the hinge pins and detach your old door for the hinges.  For most hinges you should be able to insert a nail set into the hole at the bottom of the hinge pin and strike it upward with a hammer. Keep driving the pin upward (with the hammer) until it comes out. Score the caulking between molding and wall in order to break the seal. With a pry bar and hammer, carefully remove molding. Pry away the doorjamb, framing, and the threshold. Now you can scrape away the old caulking. You need to measure the width between the side jambs, the head jamb to the seal under the old threshold, as well as the thickness of the wall. The rough opening needs to be a minimum of 1 inch (2.54cm) wider and taller than the outside of the door system you'll be installing. Be sure that the rough opening and the subsill (the section of the floor under which the sill will rest) is level. If needed, use shims or a beveled board to level it. If the door needs to clear an especially high floor, like a thick carpet, a spacer board may be needed. While you're working you should periodically check with your leveler that everything is staying level. If things aren't level you might end up with a tilted door or frame, which will cause problems later on. You particularly want to check that the hinge side is level.