If unable to obtain 3/32" thick Masonite, you can get 1/8" thick Masonite at most hardware stores. Cutting to fit your door pattern can be done using a jigsaw and fine plywood cutting blade.
For your template, either get a large piece of brown warehouse wrapping paper or large sheets of poster-board and tape the poster-board together to make a sheet large enough to cover your door panel surface. (the poster-board is a much more stable selection).
Cut out a "template" using your door inner surface as a pattern. You will only need one, since the opposite side door is a mirror image of the final template.
The first cutouts you need to make are the inside door handle and window-crank shafts. Measure from bottom of door and front or back of door to center of shaft and mark that distance on your template. Cut out a 1/2 inch diameter hole in your template for the shaft, then hold your template up to the inner door and see how the template fits the shafts while checking bottom and sides of the template for alignment. If your hole is a little bit off, you can cut it slightly larger and refit the template to the3 door again. Ultimately, the final holes for the door handle and window crank shafts need to be big enough for the backside of the door handle to poke thru, so the hole is much larger than the shaft, but that only aids in your ability to fit the template on the shafts.
Once you get the door handle and window crank shafts fitted thru your template, tape the template to the inside of the door temporarily with masking tape that can be easily peeled off later. Take a small ballpeen hammer or similar surface and RUB it all along the outer edges of the template where the mounting holes are. Doing this will expose the location of the holes in the door by making a round mark on your template.
Remove your template from the door and lay your template on the Masonite to be used as your door panel. Align your template PERFECTLY with your Masonite cutout.
CENTER-punch the holes for your door handle and window crank. Use a nail and hammer to punch thru your template to locate the CENTER of you door mounting holes but do not allow the nail to poke thru the Masonite. This is only a reference point. Once all holes are located remove your template and save it for the opposite side door.
Use a hole-saw in an electric drill to make the large holes for the door handle and window crank handles. Hole-saw size must be large enough to allow the extended portion of the back of the door handle, but small enough to cover the door handle tension spring.
Now measure your mounting clips from center where the clip pokes thru the hole in the door to where the clip fits snugly into the hole you will make in the masonite. For example, I have a bunch of OEM clips that measure 1/2 inch exactly from center of punch-in to EDGE of hole you need to make.
So, measure from center of nail-hole on your Masonite over 1/2 inch and mark the EDGE of your holes for the clips. You can use a hollow punch set to punch these holes thru the masonite. NOTE: The OEM clips are for 3/32 inch thick Masonite which is not readily available. If you got 1/8" thick masonite, it may be necessary to gouge out 1/32" on the outer face of the Masonite so that your old clips will fit in the masonite. If clips are bent, try to restore or get new clips. Make sure that new clips have the same CENTER-to-EDGE measurement. I have seen some replacement clips with a LONGER C-to-E measurement and if those are used, there may be problems aligning your new door panel with the door structure.
