Step 2: Make the rails
The sizing of your rails may vary with your table saw. The grooves in the SawStop table saw at the MPK Tech Shop are slightly narrower than 3/4", and about 1/4" deep, and somewhere between a third to a half of the overall length of your table. Measure your grooves and rip cut two strips of wood off the end of your 1x12" board that are thick enough to sit just below flush to the surface. If your rails are too high, your jig will rub on the bottom of the groove rather than sit flat on the table.
I ripped the strips off using the table saw with the standard blade and tuned them to the right width and height on the sander.
Ideally, your guide rails should be at least about 1/3 to 1/2 as long as your table. Too long and you'll have too much resistance, and you could potentially hit something on the other side of your table. Too short and the guide rails won't do their job. However, the length is not as important as the height and the ability to flow smoothly through the channels.
Before moving on, take a minute to slide your guard rails back and forth and side to side. They should glide easily along the groove (sanding will assist with that too - the guide rails will be smoother)