Drill a hole on the side of the plastic barrel large enough to fit the outdoor water spigot into snugly. The hole should be about one inch from the bottom edge of the barrel.
Secure the water spigot to the barrel using waterproof caulk and a caulking gun. Allow the caulk to set for a two to three hours. The caulk should set for approximately 24 hours before actually using the barrel to collect rainwater.
Raise the plastic barrel off the ground on two or more concrete cinder blocks. The barrel should be placed below a roof downspout to collect rainwater. Remove the lid of the barrel to allow water to fill it up. If the barrel does not have a removable lid use a jigsaw or Dremel rotary tool to remove a section of the top directly below the drain pipe large enough for water to pour into the barrel.
Connect a garden hose to the water spigot in order to water your lawn or garden.