Disclaimer out of the way, let's get started!
This how to will not include the gutter system, I figure that it's pretty variable depending on the scenario, but the barrel itself is easy and approachable by everyone.
Step 1: Materials and Step 1
My choice is a 200 L bioprocess container. It used to hold glucose.
You can acquire these barrels from various sources. I got mine free from a local manufacturing company. They throw things like this out or back into recycling all the time. Another viable option is a local carwash, they have large barrels for detergent that they need to dispose of all the time. In any case, where ever you get yours, wash the heck out of it before you start watering your food crop with it.
You will also require:
Drill bit (3/4 inch)
3/4 inch spigot union (this is going to be snug fit)
3/4 inch black connectors
standard house spigot (which should also fit a 3/4 inch fitting.
Drill the hole into the barrel near the bottom (but not at the bottom (see figure 2).
Use the rubber mallet to seat the union through your hole. This will be pretty snug I assure you. Do not bang hard, just take your time. After you've seated your union, teflon tape the threaded bits.