At some point before completing your raised bed, you'll want to provide further braces. This is as good a place to discuss it as any, although you could have installed the additional bracing as early as Step #5 (right after finishing the framing units).
The bracing that I'm referring to will serve two functions. First of all, it will solidify the framing units. Secondly, it will provide support for the flooring. The latter is especially helpful, since the amount of soil in a planting bed of this size will weigh a significant amount.
Begin by holding a 53" 2x4 parallel to the floor boards and toenailing it into the legs, spanning the length of the structure (see photo above). Repeat on the opposite side.
To proceed with this step in the easiest possible way, it's probably best to flip the raised bed upside-down at this point. Now attach three evenly-spaced 16" cross-braces (also 2x4s) to the two boards you just toenailed in, spanning them. The attaching can be done with lag bolts (using a ratchet wrench), which afford extra stability. In fact, now that you're finishing up, it wouldn't hurt to go back and throw a few lag bolts into some other key areas of the framing.
While you've got the raised bed upside-down, install a caster on each of the four legs. The bigger the caster, the more weight you'll be able to move around.
We're almost done! The project begins to take on a more horticultural aspect in Step #9....
