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.