Once upon a time, I crocheted a River Tam vest, using this great tutorial. Unfortunately, this only gives explicit step-by-step instructions for the two square side panels, and the second half, which describes joining the two squares and crocheting the rest of the vest, is a little more vague. When I made mine (once upon a time when I was a beginner crocheter...) I spent quite a while squinting at screen shots and muttering swear words to myself, and had intended to type up my notes at some point to hopefully help out anyone else who might benefit from more step-by-step sort of instructions. Well, nearly two and a half years later, here we are...
To join the two square panels, I crocheted an extra three rows on of the panels, then joined in the following manner. After completing the last row of (tr, ch 1) on Square A, line up the two panels. Treble stitch into ch1 space on Square B, slip stitch into next treble stitch on Square B, then treble stitch into ch1 space on Square A and slip stitch into top of next treble stitch on that panel. Repeat (tr in ch1 space, slip stitch onto top of next tr), alternating panels, all the way down. Doing this, I ended up with a nearly invisible seam.
Next, I added the fishnet (ch 7, sc into center of loop below, rep across) across the top of the two newly joined squares. Using ch-7 loops, I worked it for 12 rows before splitting for the shoulder sections. Each shoulder section is 6 loops, and I worked each shoulder for another 12 rows before joining to the front of the vest as follows: Ch 3, sc into corner of front panel, ch 3, sc into center of next loop. Rep across until all six loops have been joined to the front panel. Repeat on other shoulder. (joining, of course, added an extra row)
Then I added the bottom edging. Using the same ch-7 fishnet pattern, work 12 rows across the bottom of the two joined squares. After 12 rows, work bottom edging as follows: work 2 treble crochet stitches into first loop, ch 3. Repeat across. Work 5 rows total in the pattern, working the 2tr into the 2 tr of the previous row.
The front edging on the vest is the same as the bottom edging, and is also worked for 5 rows. To set it up, work 2 tr into the top corner of the panel by the shoulder fishnet section. Ch3, skip next two stitches, 2 tr into next 2 chain spaces, ch3, skip next ch space, 2 tr into same next ch space, ch 3, 2 tr into next two ch spaces. Rep across, alternating like this to get the spacing for the edging right. I'm afraid this isn't too clear; I'm not very good at pattern-writing, so here's a picture to help explain.
You can see how every first 2tr is worked into the same ch1 space, and every second is worked with one treble stitch in its own ch1 space. When you reach the fishnet section along the bottom of the vest, work 2tr into each side loop, and along the bottom edging wherever it naturally falls, fudging a bit to make it fit right. Repeat along other front side, and you're done!
Hope this helps. :)