This design has a few features that distinguish it from others. One is the height of the barrels. The height allows me to water my garden without having to do it the old-fashion way (filling and carrying watering cans).
Another distinguishing feature is the automatic overflow system. When the barrels are full, the rain continues to cycle through the barrels, but the overflow is sent through the gutter downspout. Other designs may have some kind of manual over-ride, but this design is completely automatic.
Step 1: Build Platform
When I built the platform, I thought long and hard about safety. . Making a safe platform isn't an impossible task, but 800# (density of water is 8 #/gal) is enough to be careful about. Several things to keep in mind are the strength of the decking material, the height of the tower compared to it's length and width, and the strength of the legs.
I used standard 4x8 retaining-wall block for my first attempt. It was tippy and I took it down immediately. My second attempt was pairs of concrete block, alternating. The second time 'round, I decided to use sand as the base material for the legs. It made the whole job much easier and the cost of the sand was small.
I made the platform out of bits of lumber from an old deck. Using re-claimed wood is cool, but this may not be the place for it. My deck turned out strong enough because I used two layers, but I recommend using decent wood.
Step 2: Connecting the Barrels
Join the barrels near the bottom---maybe 6" (10 cm) off the deck. The higher you join them, the more volume will be wasted in the second barrel. Any water that's below the level of this connection won't drain. Same goes for the spigot.
I was told that I should use galvanized pipe with standard pipe threads. Galvanization helps avoid rust.
One possible combination of fittings is closed nipples (short) and pipe nuts. But I couldn't find pipe nuts, so I used couplers instead. Or, that was the plan. Turns out that I was able to use standard spade bits to drill the holes and then I just screwed the nipples into the plastic. They cut their own threads and produced water-tight fittings. I think 3/4" for 1/2" pipe, 1" for 3/4 pipe. Then, you'll need a union to connect the two barrels.
Step 3: Installing the Spigot
Step 4: Automatic Overflow System
I had a PVC elbow and some pipe laying around, so I used that to connect the overflow spout to the downspout of my gutter system.
Step 5: Connecting to the Gutter
I used string to tie down the free end. I don't think it does any good at all.
I added a bit of landscaping cloth to act as a filter. After the first rain, it looked as though the water hand backed up high enough to run off the edges. I may have to try something else. I attached it with self-tapping sheet-metal screws. I put the cloth over the heads of the screws and tied it there with string.
Step 6: My Results
The water color was quite brown, which I think is due to bits of leaves that colored the gutters last year before I cleaned them out. It's kind of gross, but the tomatoes don't seem to care.
Yes, the gutter had a lot of maple seeds and flowers from this spring, which was the reason for the color of the water. I was surprised at the amount of stuff up there because I'd cleaned them out several times last fall. Turns out it was all fresh from this spring.
I tested the overflow while cleaning the gutters. Works well engough, but a static load is probably not a good idea. I had the PVC pipe hanging on the overflow output, which was enough weight to bend it down a bit.
I decided to give up on the landscaping cloth for a filter. Not only did it not work, I had mosquitoes hatching out in just a few DAYS! Instead, I got mosquito-proof screen. That keeps them in if they're in and out if they're out.
After using my rain-collection barrel, I would recommend you look for more clever plumbing. I've seen some really good designs that eliminate the amount of water that's left in the barrel. I have 10-20 gallons that I can't get out, Also, my capacity is lower that 110 gal because the one barrel was cut off (in a previous life it was an in-garage compost bin for several winters).
Also, use a 3/4 spigot if you can manage. When the water gets low, it takes almost 45 sec/gal to fill a watering can.
After going to some pains to have a gravity-feed system, I discovered that I like watering plants with a watering can because I have a better idea of the amount of water the plant is getting. It is more work, but given my sedentary profession, carrying around a few gallons of water each day is probably good for me.