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Measure for a 1/4-inch reveal at all window and door jambs. Trim to jamb joints can be difficult to line up consistently and smooth out. It is better to accentuate the joint rather than try to hide it. Hook your measuring tape at the inside of the jamb and make a mark at the 1/4-inch line. Mark the sides in three or four places with one mark at the top corner of the jamb and one at the bottom. Mark the top (and bottom for windows without windowstools) in a similar fashion. Determine the trim length by measuring between the reveal lines, and not the jamb itself. The measurements between the lines will represent the "short point" of the 45-degree miters that form the corners.
Measure baseboards by butting your measuring tape into the corner about 2 inches up from the floor. Butt the hook into one wall and the tape box into the other and add the tape box depth to the indicated number. This represents the "long point" of the 45-degree miter in the corners. -
Mark the cuts on your trim with a razor knife. Knife nicks are narrower than pencil marks and provide for more accurate cuts. The measurements for door and window casing are for the short points, so mark them out, then lay the trim onto the miter-saw deck. Lower the sawblade without starting the saw and sight along the blade to place it precisely onto your mark. Raise the saw and start the motor. Make the saw cuts decisively and with one smooth motion. Always cut trim face up to avoid splintering on the side that will show.
Cut baseboard standing vertically against the miter-saw backstop with its face out, or toward you. Mark the long-point measurement on the top edge with a small knife mark and align the sawblade and the mark as you did for the casing cuts. -
Dress the casing trim to the reveal lines on the jamb. Place the header (top) and one stile (side) into place and nest the corner joints together. This will locate the stile and insure that the header will work in that position. Remove the header while holding the stile in place, then nail through the casing into the jamb and into the door trimmers within the wall framing. Do not nail too close to the edge or you will risk blowing a large splinter out of the edge and ruining the casing. Do not angle the nail in too much, or the nail will come out of the face of the jamb and create more work to reset the nail and putty or sand the hole smooth.
Door and window jambs will sometimes stand proud of the wall they are mounted in, creating a gap at the toe (outer corner) of the 45-degree miters. Cut a small, shallow wedge and slip it diagonally between the wall and the back of the corners, catching both trim pieces. This will pull the toes up tight while creating a slightly larger caulk joint at the outer edges. Since the sides aren't as visible as the face and are easier to caulk out, this will help camouflage the out-of-flush condition.
