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.
Step 2: Step 2. Putty
In the first shot, you'll see a black ring that has been fitted over the threaded spigot union. Before this can be threaded on there, the union needs to be generously treated with the plumbers adhesive (comes in a small metal tin, yellow in color USE GLOVES!!)
After you've fluxed your union, apply the plumbers putty liberally to the two black rings as seen in figure two of this step.
Then firmly screw the two apposing rings toward one another until they lock against the side of your barrel. This is going to squish your putty out pretty good. As soon as it has, pull off the excess, and reflux the outsside of the union rings with a little more adhesive. Fit your spigot and wait.
Let dry. For a while. It's important that this be allowed to set for > 45 minutes. Got get a beer and wash your hands of the putty.
Also while you're gone, you should acquire a minion. Who doesn't like a good minion?
Step 3: Step 3. Get a Minion.
If you can get the barrel to > 1/4 full with no leaks, you are done. Make sure your spigot works. Hurrah! You're done!
If it leaks, (which it shouldn't!) you need to reverse. Pull off your spigot, then pull your putty'd rings off and reseat with new putty and new adhesive.
Good luck. This whole project takes less than 2 hours including wait time and hauling it up to the hill. Mount your unit onto something like concrete blocks (or even a small deck if you're tremendously handy) The top of the unit should be secure and you may even consider adding fine mesh to the inlet to prevent excess mosquitoes (I've even heard of people rocking solar powered aerators and gold fish to combat that problem.)
When attached to my small 8 x 7 ft shed with a gutter system, this 200 L barrel will fill with a single decent rain shower. (Not pictured is my small gutter system, I have no pics of its assemblage.)