I built a small raised garden bed: roughly 3 feet by 3 feet in the interior (with 2' in wall height, achieved via two 2x12s). Just a bottomless planter box, really. For larger raised garden beds, the same instructions apply, only you'll need more lumber (and you'll have to adjust, accordingly, all the measurements I furnish in my tutorial).
However, 4 feet is recommended as the limit on width for raised garden beds, as you want to be able to reach easily into the middle from either side. Dimensions of 12 feet long by 4 feet wide would have made for a more useful raised garden bed, but I just didn't have the room for that in the small space in which I was working.
Here's the basic idea behind my project. The raised garden bed will consist of four pre-assembed walls (hereafter referred to simply as the "walls"), each of which is composed of two 2x12 boards screwed onto two 4x4 posts (hereafter referred to simply as the "posts"), one post for each end. See picture above. The extra 2 feet of post will be sunk into the ground, in order to stabilize the raised garden bed. After the walls are assembled, they will be brought over to the site of the raised garden bed. Holes will be dug in the ground, and the posts will be inserted into those holes. When the posts fit just right, braces will be attached to the walls, linking them together. The final steps will revolve around packing the dirt back into the holes (to secure the posts), filling up the raised garden bed with soil, and attaching the cap boards.
If you're going to apply a preservative to the wood for raised garden beds, it's best to do so at the beginning of the project. Let it dry before pre-assembling the walls. For raised garden beds meant for vegetables, stick to natural preservatives, such as linseed oil.
In Step 5 we'll begin pre-assembling the walls of the raised bed....