I already had a steel bar 1/4 inch x 3/4 inch for making the miter gage. 3/8 inch x 3/4 inch is the usual size of a bar for a miter gage. I needed to make a slot in the table to fit my steel bar. Make the slot edge nearest the blade about four inches from the blade. The easy way is to use a table saw or radial arm saw. A dado head makes it very easy. In the absence of those things, a circular saw, a framing square, a couple of clamps and a precise rule will help you do a decent job, too. Set the saw depth to the thickness of the bar. Clamp the square at a right angle to the front of the saw table. Use the square's edge as a fence and guide. Make a cut. Use the edge of the cut to measure so you can move the square laterally almost the thickness of the saw blade. Make another cut. Keep the cuts parallel to the first cut. Make the last cut so it allows the steel bar to move smoothly in the slot, but without looseness side to side. Finish the slot with a file or scrape smooth with a chisel. (The second clamp on the square is outside the right border of the photo.)