I'm very visually-oriented. So I like to lay out the pieces, where possible, before doing any joining in a project such as this one. So before building the corners, we formed a temporary corner by laying out the landscape timbers (one 4-footer and one 8-footer, placed so as to form an elbow) in the corner area. The string and stakes were still in place, so that we could double-check ourselves.
This is an especially helpful step for those of us who are absent-minded. For example, it would have been easy for us to forget that we needed to build the corner in such a way that it would come out 4 feet 3 inches away from the fence (measuring to the outer face of the landscape timber), rather than just 4 feet. Exactly how you join the two landscape timbers together to form a corner determines whether or not you stay true to that measurement.
Once you've verified that you are, indeed, on the right track, take the two landscape timbers back out to a spot where you can work with them more easily (assuming you have plants in the way in the border planting, as we did). Now use corner braces to join landscape timbers together to build a corner (picture). Galvanized corner braces are preferred, since they hold up better to the elements.
With the landscape timbers situated so as to form an elbow, we laid the corner brace in the "crook" of the elbow. To double-check that your landscape timbers are meeting at a right angle, use the carpenter's square. Then, using a drill, we simply screwed the corner brace into place. But if you encounter any resistance (as you may if you hit a knot), you can pre-drill a pilot hole.
On the next page, we'll walk the newly-built corner piece over to its final resting place....