If you have a vent window, you will want to trial-fit its frame and adjust the frame-to-skin gap. You can use a dial caliper to measure the desired distance. Adjust the gap by either pushing out or pulling the doorframe and skin together. You can use a strap around the door to hold the gap in while you work. The two upper braces on the new skin will overlap the old braces attached to the doorframe. Once the desired gap is held in place, drill a hole and insert a sheetmetal screw in each brace. If you need to make further adjustments, you can drill the top hole bigger, then when you break through, switch to a smaller bit. This allows you to loosen the screws and reposition them. When you're ready, tighten the screws to secure the braces.