If you already know that water won't run uphill and how to handle a drill, you can do this. The most ingenious part of the design is the hose-to-barrel connection. Since the attached hose will frequently be tugged during normal use, it is important to use a mechanical connection rather than a glued connection.
This design uses a simple garden hose washer, standard garden hose parts, and a special adapter, that's not really all that special. It's expandable and useful for more than just capturing rain water.
Step 1: Got Parts?
Also, while in town, pick up a garden hose valve, garden hose washer, and a MHT to FPT 3/4" plastic adapter. What? MHT = Male Hose Thread. FPT = Female Pipe Thread. Plastic = plastic. You'll know it when you see it. Less than $10 total.
I had some old parts handy to draw from.
The gutter modification comes later.
Step 2: Tools Needed
To modify a gutter downspout, you will also need a heavy duty pair of scissors (or sheet metal snips), about 10 small sheet metal screws, and a small drill bit to pre-drill for the screws. A pop-rivetor would also work. The special pliers shown in the photo are for "shrinking" sheet metal. Optional.
Step 3: Saw an Opening in the Top of the Barrel
Step 4: Attach Garden Hose Valve
Step 5: Modify the Downspout
The parts photo shows a flexible plastic downspout that may also work.
Step 6: Add More Barrels
I learned that one 55 gallon barrel can fill up in about 10 minutes of heavy rain. The secondary barrel would only partially fill in that time due to the relatively small diameter of the garden hose. Adding two or three or more secondary barrels, each with its own dedicated fill hose from the main barrel, will work much better, saving more water during those times of heavy downpour.
Make your hose-barrel connections the same way for secondary barrels. The second photo shows a cut-away mock-up of the hose-barrel connection. In this case, a quick-release brass connector just happened to work for the inside job. However, the MHT to FPT adapter is better since it has a larger diameter. A different garden hose valve did duty for the outside job.
Step 7: Enjoy Free Water!
As a bonus, this system makes a splendid watering system for gardens during dry seasons. I use tap water to fill the barrel with 15 or 20 gallons, and then use the barrel outlet hose to meter out water to specific garden areas. You can walk away from it, and your garden will slowly receive that 15 or 20 gallons, no more, no less. Very nice for those times when rain water is in short supply.