It's pretty straight forward, cut that hard plastic bit off the rear facing side of the door panel, peal back the leather on the 300 door panel, carefully. match up the intrepid peice with the 300, mark and cut. Then using a solder iron, you fuse the plastics together.
I suggest keeping strips of plastic around that you can melt like goop to fill any holes, and create more surface.
Really, it's more of a craft than a procedure. You will have screw ups, and frankly I never seen a set that looked 100% factory.