How to install an exterior door
You don't need to be a carpenter to install a door
Installing an exterior door is similar to installing a window. The frame and door are generally prefabricated and sold that way through home improvement stores like Home Depot or Lowes. Some of the tools you'll need are a pencil, an electric drill, a level, utility knife, shims, insulation, phillips head screwdriver, circular saw, hand saw, building paper, a drip edge, and safety glasses. If you're doing many home improvement projects you'll want to have most of these things anyway.
Setting things up for installing a door
First, you want to put the door in the opening to test fit it and use the level to check it. Trace the exterior of the door molding on the exterior of the house with your carpenter's pencil. Trim the siding of the house with a circular saw along the line you just traced. Go slowly to make sure you get it right. You don't want to mess up this step because you were going too fast. Use a wood chisel to get the corners just right. Now slip building paper in between the siding of the house and the sheathing and wrap it around and staple it to the studs on the inside of the house. You'll want to install a drip edge along the top of the frame. Use silicon caulk to seal in the area between the building paper and the siding along the frame of the door.
Installing the door
Now put the door and the frame in the opening. Insert shims from the inside part of the door about every foot on the side of the door frame between the frame and the opening. Do not insert shims on the top or bottom of the door. Check to make sure the door is level. If it is, you can start stapling the door frame to the opening. Make sure to staple at the shim locations.
Your door and frame probably came with the hinges installed and door already attached. Chances are it probably also came with anchor screws. Remove two screws from the top hinge and insert the anchor screws in their place.
Finishing up your door installation
Now you can staple your door frame to the molding along the outside, caulk between the frame and the siding, screw down the threshold on the bottom, and use your utility knife to cut down excess shims. For more detailed instructions, see the video below.