Step 1 - Make a level spot under the gutter downspout for the barrel to sit on (Make the base strong, the barrel can weigh 600lbs full)
Step 2 - Move the barrel in front of the downspout. Mark the downspout right above the barrel with a marker or pencil.
Step 3 - Place Something (like cardboard or plywood) behind the downspout to protect the siding and cut downspout on the mark you just made.
Step 4 - Move the barrel in place with inlet under the downspout.
Step 5 - Use a hose clamp to attach a hose to the overflow fitting. You can cut the end off an old leaky garden hose and get a second life out of it.
Step 6 - Overflow hose can be placed in a garden to help redirect the water to a helpful place when the barrel is full
Now that the barrel is install open the faucet and fill the barrel from the top with water until water starts flowing out of the faucet. Stop filling the barrel and try rocking the barrel back and forth well checking that it is stable and safe. Fill the barrel all the way up and recheck that it is stable and safe. Rain barrels are great for watering the garden but not for drinking or bathing.