We'll assume you already know the importance of catching rainwater off your roof so it doesn't hit the sewer system. If you water your plants with it, they will thank you, too!
You can get a rain barrel kit that would include the barrel and all the necessary parts, but I would warn against using plastic or cheap spigots or other pieces that break easily. Each barrel is different and you can customize each one to catch and hold rain. The essential elements are these:
Hole or holes to allow rain into the barrel
Screen to keep out debris and mosquito larva
Overflow exit at the top, which allows the same amount of water out as what is coming in
Placement on an elevated surface so you can fit a bucket under the spigot and to increase the water pressure for use with a hose
Here are Omaha Stormwater's instructions. I've used them and tweaked a few things.
You can get a sustainable, repurposed barrel on Craigslist, or simply call or text Chris at 402-881-6919 for a free one. He's a cool guy in town that knows a lot about native grasses, helps out with Omaha Permaculture, and finds new homes for barrels used by the city's Public Works Department.
You can follow the instructions below, or simply drill a 3/4" hole so the threads fit tightly. Perhaps use teflon plumber's tape or the sealant described below on the threads if you're concerned. There are many ways to make this work.
If you'd like to use the washers, you'll need:
heavy metal spigot, with nut (not a lightweight one as they break easily)
2 metal washers
2 rubber washers
watertight sealant (the kind recommended for outdoors and for use on plastics)
drill and bit
Drill a hole about 2" from bottom of barrel, just big enough for the spigot to fit tightly. Omaha Stormwater recommends placing a metal washer, then a rubber one, onto the spigot before screwing into the hole. Then on the inside, rubber, then metal again, then the nut. This only works if you have a removable lid on your barrel and would not all fit with the spigot I purchased. I used teflon plumber's tape on the spigot threads instead of the rubber washer. Use sealant on both sides of the spigot where it meets the plastic. Once the sealant dries thoroughly, your spigot is ready!
There are multiple options for water entry, but I like the one offered by Omaha Stormwater. The basket allows a space to collect and remove debris from the roof and empty into the yard and to see the water level easily. I've also seen holes drilled straight into the lid with screen secured to keep debris out. The UNL Extension Office recommends using a strainer, which would work, too. For the basket version, you'll need:
a fish pond basket (wherever fish supplies are sold)
plastic window screen (Can we share? You have to buy a lot of it at once. I don't have any currently, but if you have some to share, please post in comments!)
jig saw, utility knife, or hand saw
heavy duty glue to secure screen to basket
Cut a hole in the lid big enough to hold the basket and small enough that the basket won't fall through. Cut the screen to fit inside or outside the basket. I glued the screen to the outer edge of the basket.
This can be done in a multitude of ways, but you want the water to leave the barrel just as fast as it's coming in, unless you don't care if it spills onto your surface and it wont' back up into your downspout. You can attach more barrels to the first one if you'd like to retain more water. PVC piping can be attached at the top, diverting water to the ground or another barrel, holes can be drilled into the side of the barrel near the top for water to escape, or a diversion inline can be installed to divert the water once the barrel is filled.
A filled 55 gallon barrel weighs about 450 pounds, so you want it to be very secure in its location. Concrete blocks or something similar can be used to elevate the barrel. Downspout elbows and extensions can be used to move the water to wherever you want it to go. Of course you can paint it to match your home or trim for a nice touch.
For more information on water conservation, stormwater management, and rain barrels, visit the Omaha Stormwater Program and the UNL Extension Office. Questions? Comments? Please voice them in the comments section. Andy Szatko of the Omaha Stormwater Program will answer questions on FB!