Leaving the first wall in place (braced up with temporary supports, if necessary), bring over another pre-assembled wall and hold it up against the first one, so as to form a right angle. This will help you visualize your next step in establishing your raised garden bed frame.
For, if it hasn't occurred to you already, at this point the idea should occur to you that you don't have to dig eight holes for the eight posts: you have to dig only four -- two posts to each hole. But you should now also be able to visualize that the holes will have to be wider than you might have thought, initially (to accommodate that second post).
Now that you have some idea where this second wall should fit, begin digging accordingly. This means widening one existing hole and beginning a new hole. Continue to check for fit, as you proceed, by inserting the posts of this second wall into their holes. In the same manner, work on adding the third and fourth walls. Check for plumb and level as you go along.
I encourage you to continue bringing over your pre-assembled walls, to check the fit, throughout the construction of the raised plant bed: a visual model is preferable to abstract measurements for most of us!
In Step 8 we'll jump ahead to the point where you have a reasonably good fit for all four walls....