Step 5: The Base, step two - Routing the grooves
There are a number of tricks to using a router. First, the bit spins in a clockwise direction, as you look down at the router from the top. This means that when you cut with the router from left to right, the bit will tend to pull the router away from you, and when you route from right to left, the router will pull towards you. So, if you're hooking the edge guide along the near side of the board, route from left to right, and when you're hooking it along the far side of the board, route from right to left. Second, always test the position of your bit on scrap material. Your odds of getting it exactly right by eye are nil. Third, don't cut more than 1/4" deep on a single pass. We want a 3/8" deep grove, so make your first pass at 3/16", or thereabouts, and make a second pass to reach the full depth.
I ended up making a number of practice cuts. The first revealed that I hadn't tightened the screws on the edge guide enough. The second revealed that the design of the edge guide provided very little support at the end of a board, because of the cut-out for the router bit. In the "Getting Started in Woodworking" video, they had screwed a piece of hardwood to the edge-guide, to provide a continuous -- and longer -- bearing surface. I may do that myself, some day, but I didn't have the materials at hand, so I clamped some 2x4 scrap to the end of each board, to provide a continuous bearing surface past the ends. The two grooves in the long stretchers and the side groove in the short stretchers have identical layout. I made practice cuts in scrap until I had the edge guide set correctly, then I cut them all with that one setting. The bottom groove of the short stretchers uses a different setup, so it was back to the scrap, before cutting them.