Glue up the trim on the end, first. Do a dry fit, first, then as you take it apart lay everything where you can easily reach it as you put it back together again, after adding the glue. To help keep the edge piece aligned, I clamped a pair of hardboard scraps at each end. I used the piece of doubled MDF I'd cut off the end as a cawl, to help spread the pressure of the clamps. Squeeze some glue into a small bowl, and use a disposable brush. As you clamp down, position the trim just a little bit proud of the top surface.
In the "Getting Started with Woodworking" video, the holes through the 4x4's were drilled from the back. That is, they start on the side opposite the precisely-positioned mark, and drill through to hit it. If they can do this, more power to them, but I can't drill through 3-1/2" of wood to emerge at a precise mark without a drill press - and not always then.