Step 4.5. Fixing the muntin bars
I lay one of the door/window units flat on a working platform.
I marked around the inside of the frames 10mm up from the bottom.
I fixed muntin bars around the inside of the frames on that mark, i.e. the muntin bars were 10mm up from the bottom.
The side (vertical) muntins had the notches facing down, and the top and bottom (horizontal) muntins had the notches facing up.
I cut a 3/4" (18mm) plywood platform slightly smaller than the size of the muntin grid area, say approximately 21½" (537mm) x 59" (1475mm).
I placed the platform in the opening underneath the muntin bars so that the muntin bars were sitting on the platform.
I joined (with glue) the rest of the muntin bars (both vertical and horizontal) required to make up a complete grid.
The notches in the horizontal muntins slotted into the notches in the vertical muntins.
I then made up a second grid and sat it on top of the first grid with no nailing whatsoever, just plenty of glue in the notches where the muntin bars joined.
I separated the two grids with some plastic so that the two freshly glued muntin grids wouldn't stick to one another.
That was the bits and pieces for one door/window unit taken care of.
I then did the same for the other two.