Replacing Door Thresholds
Many exterior doors have a threshold that the door closes against. Because they often get stepped on, thresholds (which can be wood, metal, or even plastic in the case of some that incorporate weather stripping) can become splintered, gouged, or broken. Luckily, this is a relatively easy fix.
Measure the threshold that's currently in place, and check to see how it meets the door frame (its edges might be cut straight across, or they may have a cutout to allow the threshold to fit around the door frame). Also measure the height of the threshold. Choose a replacement threshold that can be adjusted or cut to fit, or is already the correct width and height. To test the fit, hold the threshold in place and have someone gently close the door. Make any width or height adjustments as needed.
Follow the instructions that came with your threshold, if applicable, to insert it in place. Generally, you'll be screwing the threshold to the underlying door frame. Metal and plastic thresholds have predrilled holes; with wood thresholds, you may need to drill the holes yourself, to avoid splitting the wood.