You now have the walls of the raised bed pre-assembled, so you know what lengths they measure; now all you have to do is carry over those measurements to the site where the raised bed will stand and dig your holes, accordingly. In my case, that meant beginning with two holes about 3 feet apart. Remember, the idea here is to sink the posts into holes, to stabilize them.
I began with the idea of digging my holes 2 feet deep (as I'll soon explain, though, I couldn't quite achieve that depth). Why 2 feet deep, specifically? Submerging half of the post's length would provide excellent stability. And that's precisely how much post I had left over at the bottom, after attaching my 2x12 boards to the tops of the posts.
Once you've removed a decent amount of dirt from the two holes, bring one of the pre-assembled walls over and check to see how its posts fit into the holes. When you begin this checking process, you'll find it handy to have old lumber around, to use as temporary bracing (so your walls don't fall on you!). Once you have the two posts in the two holes, check to see if your wall is plumb and level. You'll also be better able to visualize now just how much more digging you'll be doing in these first two holes.
But don't finish the digging in those first two holes at this point: for now, you're just trying to get a feel for how everything will eventually come together. The raised bed really starts to take shape in Step 7, when we begin digging for the second wall....
