Measure the threshold and the trim twice before cutting. Use a t-square to draw a straight line across the trim. Use a washable marker to mark the line in case you have to erase it after the second measurement.
Place the trim on a work table. Rest the longest portion of the trim on the table and hang the short edge off so that the cutting mark extends 2 inches over the edge of the table. Clamp the trim to the table once it is in place.
Grip the hacksaw by its handle with your dominant hand. Extend your index finger along the side of the top of the handle so that it points in the direction of the hacksaw blade. Place your other hand on the outer edge of the hacksaw's frame near the wing nut that holds the blade in place.
Cut along the mark made in Step 1, using long, quick strokes that utilize as much of the blade as possible. Put more pressure on the forward motion than the back-pulling motion.
File away any shavings from the end of the cut edge.