
:) Our rain barrels are set up! :)
Seriously, after laboring for a few years over the best way to do these, they're really easy to assemble, now that I've worked out all of the kinks.
In fact, for anyone interested in making your own, go HERE for probably the best instructions I've seen.

The *ONE* thing the link doesn't address is linking multiple barrels. That was the biggest struggle I had.
My conclusion about overflows is pretty simple. You gotta have the primary barrel higher than any of the others, as you can see in the above photo. Your barrels that will store are lowest. The end barrel has to be a little lower than the primary barrel. The idea is that the excess water will flow down from the main one and deposit in the middle and flow through to the last one. I do not have an 'overflow' on the main barrel or second one. I have that on the last one only (it's more easily seen in the top photo here, though).
A few more tricks:

I didn't want my barrels on the side of the house where the downspout was, but around the corner. So I used the flexible downspouts. If it gets really windy, they won't stay, so I use bungee cord to secure them in place. This is really a key element for me.


This is my overflow barrel. The second in line. I used 1.5 inch PVC to go from the primary barrel to the final one. Those are connected through the side. Any barrels in between are not. They simply have the spigot in the bottom and mesh on top. The joint is moveable and when the barrel is full, I can move the entire elbow mechanism into the UP position so water flows to the last barrel, but not into the ones in the middle. Does that make sense?