The idea in this project is to match the existing door jambs and decorative moldings in the rest of this room. If you are matching other elements in your room, just measure those pieces and transfer their dimensions to the area you are working on.Then measure the doorway you are working on before cutting the door jambs, plinth blocks and decorative moldings.
To ensure that you make straight cuts, use a table saw with a cutting fence for all but the shortest cuts. The decorative pieces may require a miter saw.Cut all of the jamb pieces, plinth blocks and decorative trim pieces to size.
After making sure the door framing is plumb and level, place the side jamb vertically on one side of the doorway resting on the floor.Use a nail gun with 2" finish nails to nail it to the framing, making sure it's plumb and flush to the wallsNail the other side jamb as well. Nail in the header jamb piece into place. All pieces should be flush with each other.
A plinth block is a traditional architectural element that serves as a foundational block at the intersection of the baseboard and a vertical casing. After cutting back the baseboard the width of the plinth block, set and nail the plinth blocks at the bottom of the casing trim pieces.
Next, nail up the outside casing trim. These trim boards are cut slightly narrower than the plinth block and secured to reveal about 1/4" of the side jamb. Install the casing trim. Space the nails about 12" apart (Image 1). To match the top casing trim on the other doorway, a flat 1/4" by 1-1/2" piece of decorative trim is nailed on top of the two vertical casing trim pieces and just below the top casing trim.After the two side pieces and the thin decorative trim piece are in place, nail the top casing trim in place (Image 2).
Measure the distances between the existing decorative moldings on the other door to ensure an accurate placement on this doorway. If your decorative moldings require a return on the corners, you'll need to use a miter saw to cut 45-degree angles on those pieces.Nail up all remaining decorative moldings. Use wood filler with a small putty knife and paintable caulk to fill nail holes and exposed joint lines.
Sand the wood filler when dry. Paint with a high-quality primer; let dry, and paint.