There are a few basic steps you'll need to take in order to install exterior door trim. First, you'll need to measure the trim and cut it to size using a saw. Once it's cut to size, you'll need to attach it to the door using nails or screws. Finally, you'll need to fill in the gaps between the trim and the door with caulk or sealant.

Well, it depends on what kind of trim you're installing. If it's a piece of molding that goes around the door, you'll probably want to attach it with nails or screws. If it's a casing that goes around the edge of the door, you may want to use a caulking gun to apply sealant and then attach it with nails or screws. And if it's a threshold, you'll probably want to use construction adhesive to attach it. But before you start attaching anything, make sure you measure and cut everything to size first. And always use caution when using power tools!