If you have an existing threshold, remove it and use it as a guide for cutting the new one. If you don’t have a threshold, first clean the area well, then carefully measure the width of the bottom door casing at its narrowest point. Subtract this measurement from that of the threshold and divide the difference by two, then measure and mark in from the threshold’s sides—this will keep the pre-drilled screw holes centered. Repeat on each of the separate pieces. Then, using a hacksaw or power saw, cut off the ends along your marks.