SG: if you are using keys or slides, you will have already cut the slot for the key prior to drilling the holes for the key. So laying out the location of the small rectangle that forms the slot for the key, on both sides of the stock, requires very accurate measurement. Once I have established the location and the outline of the key on the wood, I centre punch three times along the length of the rectangle, set the drill press on its highest speed, and very carefully drill the three holes to intersect with the slot in the tenon. then I turn the stock over, level it carefully, and drill the second set of holes.
I make thin chisels out of high carbon steel to cut away the wood between the holes, prior to burning the very little remainder of wood. The keys fit very snuggly in the rectangular slot ans slip easily through the slot in the tenon.