I failed to mention a few steps in the dry fit that helped keep everything in alignment. These photos are of the wider, front section of the layout, but other than the dowels and dowel holes both pieces are the same. Filler piece aligned and gently started into door. I had to gently tap on it. Here it is in shown in the section in the upper part of the photo. It is almost flush with the doorskin, just sticking out about ⅛" at the most exposed part of the filler piece. My cut through the door was not straight, square or plumb therefore the filler piece could never be truly flush. Now go to the end of the door section that the filler piece was just dry fit into. Drill two holes trough the top/bottom member of the door and into the cleat at the end of the of the filler piece. Deepening on the size of the top/bottom member you may need long screws. Even with 3½" deck screws I still needed to counter sink the screwheads about ½". I didn't realize this until after the picture was taken. (Just pretend the heads of the deck screws in the picture are really deep in the wood and the spliceplate only has 3 screws in the right half.) Repeat this at the other end of the SAME section of door. When complete both ends of this filler piece are temporally secured in place. I don't have the correct angle picture for the next step but it is pretty basic. Step back at the end of the layout and look down along the filler piece. We are just looking for it to be reasonably straight. There is no need to get crazy with a long straight edge or string line. We just don't want any major bulges or bellies. If the cut through the door is a little wavy like mine is, don't worry if the filler piece doesn't exactly parallel your cut. It shouldn't. The edge of the filler piece should however be vertical ie straight up and down. This doesn't need to be perfect either just eyeball it or if it makes you feel better check with a square. Ok, almost done the dry fit on this piece! Two more quick steps. This picture shows both pieces close together but for now we are still just working on one section. Every 18"-24" along the cut edge of the door skin tack in a small finish nail or brad. Leave plenty of shank so you can pull it back out easily. Place the nail so it will go roughly into the middle of the width of the filler strip. You can see the finish nails in the above picture. You want to do this on one side (top or bottom) of this layout section. Now take a sharp pencil and draw a mark where the door skin meets the filler piece. This marks how far to slide in the filler piece in on final glue up. Mark the finish nail holes and mark the exposed side of the filler strip with some arrows pointing up. You might want to mark which layout section the filler piece goes to. Now step back, take a deep breath and admire you're handiwork. Don't worry the filler piece isn't square or plumb. During the next update we are going to match the other layout section to the one you just dry fit. The next layout section will find the "sweet spot", kind of like spooning layout sections . Hope this helps.