How can I make a rain barrel and water the garden from it?

I got a food grade barrel used for $10, thing is huge - larger than 50 gal, and installed a spigot at the bottom a hose can be attached too. Bought spigot at hardware store. I cut the downspout off at the end of the garage so I can put barrel underneath, and have a piece that I can put on or take off after barrel is full. I have screening on the top held in place by 2 bungee cords. so no mosquitos ( my girlfriend put goldfish in her barrel).

if you can't find a barrel, there are some nice, compact styles for less than $150. Some can connect together, and have filters to keep debris out

Size of bucket will depend on how much water you will need and your areas rainfall. I am in mid Michigan and I use two 50 gallon rain barrels. It tides us over when dry spells hit. Otherwise I fill in with well water.

They sell diverter kits at hardware stores. It attaches to the downspout. We used a 50 gallon pickle barrel (bright blue plastic) to collect the rain water. Ours sits on a heavy wooden stool about 20 inches off a cement pad my husband made under the downspout. 20 inches allows room for the spigot and hose, and also room to fill a bucket or watering can. An overflow rain barrel can be added if needed . We used a washing machine hose to connect the two barrels. When the water level in the barrel on the bench nears the top, the short hose at the top of the barrel directs the overflow water into the second barrel sitting lower on the cement pad. (That one is raised up only 6 inches on thick bricks to allow the second spigot to be used). Both barrels have close fitting wooden lids and nylon window screening we made ourselves to keep bugs out. If I planned to filter it I would attach a filtering tank as well.