This plan was easy to follow, accurate and came together in just a few hours. I used some reclaimed barn wood for the 2x6s and dog-ear pickets for the remainder.
I made it harder for myself than it had to be. I didn't follow the directions closely enough and cut the box ends at 5 1/2" instead of 5 1/4". That meant that the bottoms had nothing to press-fit against. So, I had to do a little workaround to fix that. I made a 1" shelf at each end of each box so that the bottom could sit on these little shelves. The upside to this mistake is that there was now a drainage slot all along the front and all along the back of each planter box.
I stained and sealed it. Finally, I lined each planter box with weed matting that I'd cut to size and cut drain slits in. The matting doubles as a way to keep the planting mix from falling out of the long drainage slots while allowing water to drain through the little slits I poked in the fabric. (I poked drain slits only along the front edge so that water would drain into the planter box below instead of onto the ground.)
I'm happy with the plan and the finished planter!