Hi Mike,
To make the jig I use a 1/8” thick piece of Baltic Burch plywood, cut a slot halfway through and glue a strip 1/16” x with of saw kerf in the slot, cut a piece of wood to fit in the miter gauge slot in the table saw and place it in the slot, set the jig aligned with the blade and move it towards the miter slot the distance of the open space of the grating usually two scale inches (3/32” for 1:32 scale) mark the slot at that location and glue the miter board on the bottom of the jig there. I use clamps to hold the jig in place with the blade lowered below the jig turn on the saw and raise the blade through the jig about 1/8” above the jig. Using a piece of scrape the same thickness of the grating lower the blade to slightly below the center of the test piece and cut a kerf turn the test piece over and cut another kerf repeat until you reach the center. The jig is ready to go.
Make the first cut holding the grating board next to the wood fence and cut the slot, move the board over the fence and make the next cut repeat to end of grating board.
When I make grating I make enough for two or three builds, the next time I make some I will take progress photos for a tutorial and post it under the section for jigs and stuff.
By using the photos this should get you started, good luck.
Mike