Installing Door Casing With A Reveal
When you install a pre-hung door in an opening, you are going to have some nasty looking gaps and cuts. The gaps will be between the drywall and the perimeter of the door frame. This area is used to shim the door to make sure it is plumb and square. This helps the door to operate correctly.
Over these gaps is placed door trim or casing. It installs easily with a nail gun and a compressor, but you can certainly use the old hammer and 4d finish nails. The key is to have a reveal.
For door casing, a reveal is the small area that stands proud of the casing. It basically is the corner of the door frame as it is revealed from the edge of the casing. As you lay out the casing keep the reveal at a uniform 3/16ths of an inch all around the opening. It will give the installation a more attractive appearance. You can use an adjustable square and set it for this distance and then just mark a pencil line around the perimter of the jamb. This line is where the edge of you casing will stop.
Use a level and secure one side of the vertical jamb with 2 finish nails (the nail gun is perfect for this job-very quick and accurate), after cutting a 45-degree miter at the corner. I like to start on one side, cut the miters, and then continue across the top and down the other side. Once the door casing has been installed, you can install the baseboard square to the casing, or you can miter cut both pieces to make everything flow without any breaking points.
Once everything is installed, you can fill the holes, sand, and paint.