I followed these plans from This Old House with a few changes. First, I changed the dimensions to what we needed, and two, I made this out of wood, not PVC. We had some scrap beadboard laying around that I could use, and PVC was going to be waaaay too expensive anyway, especially since I wanted 3 of them (sorry, Norm...)!
We built these 3 years ago, so the details are a little fuzzy, but we basically followed the plans. I would recommend building the legs a little differently than they do in the plans if you are not going to make them out of PVC, simply because joining them with screws was a little difficult when the directions are made for PVC glue. I would make the legs out of a 2x2 or 4x4 post, depending on how big you want the planters to be.
When we put in the bottom boards (TOH step #9), we left some space between each board for drainage. Once we were finished, I lined the inside with garden fabric so that the soil wouldn't fall out.
Here is the un-finished version:
I finished this with a grey stain, which I applied lightly and then rubbed off with a rag before it dried. I wanted them to look weathered, and I think the grey stain did the trick!
Here it is after a year, and full of flowers!
... and 2 years later!