First, the pieces. These are part of a vinyl fence that was cut up as part of our initial raised bed project. Since we only had 4 pieces, this one was going to be 1/2 the size of the others at about 4' x 4'.
Next we took the 4" x 4" vinyl fence post and cut it into 17 3/4" pieces and then cut out a slot for the panels to fit into. These panels just happened to be 3/4" thick and 12" tall. This is pretty easy to do with a Sonicrafter, which is what we used, or you could use a drill and a jigsaw. Here's J. cleaning up the plastic burrs with a pocket knife. He would make a great hand model, don'tcha think?
Next, put everything together and square it up. Two panels should run long all the way through the corner posts and the other two side panels will butt up against them (see the pic below to see what I'm talking about). Our yard has a slight slope to it, so we had to dig down one side to get everything level. Once it's where you want it, cut some rebar and hammer it into the inside corner. This will keep everything together.
When you're done, it will look like this:






































