The boards shown intersecting in the picture above are the cap boards. Not only do they give the raised bed a more finished look, but they also provide me with the outdoor work bench I desire. If you want to get fancy, you can use a miter saw to put 45-degree angles on these boards.
I used 2x12s for my cap boards. Perhaps 2x10s would have been preferable. But, as I mentioned earlier, I wished to keep this project simple (and cheap). When the lumberyard cut up the 2x12s for my wall boards, it was simpler and cheaper to use the leftovers for my cap boards, rather than buying extra lumber.
Two of my cap boards are 4-footers; I attached these first, on opposite ends. I pre-drilled, then I screwed them onto the tops of the walls. They're lined up so as to receive support primarily from the 4x4 posts beneath.
With the two 4-footers attached, I simply measured between them to determine how long the final two boards must be. I cut them accordingly, then attached them.
Extra support can be provided for the cap boards by using metal braces -- corner braces on the inside, straight braces on the outside.
As previously mentioned, I lined the raised bed (walls and bottom) with landscape fabric. The black you see in the picture above is the landscape fabric. I tucked it under the cap boards, before attaching the latter.
If you have problems with gophers (or other tunnelling garden pests), this is a good time to take gopher-control measures. Line the bottom of the raised bed with a mesh (i.e., wire fencing with small holes) or with chicken wire.
In Step 10 we come to the part of the raised-bed project you've all been waiting for: filling the raised bed with soil! But soil will be only one of the ingredients....