A 45-degree miter cut placed on a piece of baseboard trim needs to be cut precisely to ensure a professional finish to the installed baseboard. It is nearly impossible to cut a 45-degree freehand. Combining a handsaw and a manual miter box will allow to produce consistent 22.5-, 45- and 90-degree cuts on baseboard trim. A manual miter box is simply a three-sided box that has angled cuts in its sides. The cuts are set on specific angles that allow you to slide the saw into the cuts and hold the handsaw at an exact angle.
Set the piece of baseboard trim on the worktable with the back of the trim sitting flat on the table.
Hook the end of the tape measure on one end of the baseboard trim and pull the tape measure along the length of the trim.
Mark the dimension that you need to cut the trim on the face of baseboard with the carpenter's pencil.
Stand the baseboard trim on end with the pencil mark facing toward you and slide the baseboard trim into the manual miter box.
Align the pencil mark you placed on the face of the baseboard with one of the 45-degree slots on the side of the manual miter box.
Slide the handsaw into the two corresponding 45-degree angle slots on the manual miter box and hold the baseboard tightly against the side of the miter box with one hand.
Use a sawing motion to cut through the baseboard with the handsaw. Do not apply excessive pressure to the handsaw, as this could result in the baseboard cracking.
Remove the cut baseboard from the manual miter box and slide the 45-degree cut over the edge of the worktable.
Lightly sand the 45-degree cut with 200-grit sandpaper to smooth the cut.