If you want to install a single rain barrel here’s one way to do it.

I’d been planning a rain barrel for a few years without executing but I finally got around to it.

The first step is to level the spot as best you can – I used pea gravel and cinder blocks.
Next, remove the old down spout – If possible remove at at joint by just removing some holding screws, otherwise you may have to cut it with a hack saw.
Measure and cut – this was not as easy as I wanted, had a difficult time getting the bend to fit the down spout.
Now re-assemble and put it back. I hope it is level enough and the bigger overflow I installed works correctly!
Add a rain barrel – The water is free and better for your plants than from the hose.