Using a saw, cut a hole on the top of the barrel to accept a gutter pipe to collect the rainwater. If using a grate or screen to act as a filter, cut the hole the appropriate size to receive the grate or screen.
Drill holes in the rain barrel to accommodate the overflow and spigot. Cut a hole toward the bottom of the barrel for the spigot. Cut a hole toward the top of the side for the overflow.
Attach the spigot in the bottom hole. Use Teflon tape for the threads. Caulk the circle to create a waterproof seal.
Attach the PVC adapter union into the overflow hole. Attach the elbow with PVC glue. Clean and glue a length of pipe to the elbow to carry the overflow water away. The length of pipe is dependent on location and how far away you wish to carry the water away from the barrel. Caulk the hole to create a waterproof seal.
Set the rain barrel on the site it is to be used. Ensure that it sits level. Choose the highest point around your house because drip irrigation relies on gravity. Cut and move the existing gutter so it enters the receptacle to receive rainwater. If using a grate or screen at the top, cut the gutter so it rests atop the screen.
Attach a soaker hose or drip irrigation tape to the spigot on the barrel. Stretch the hose or tape to snake around the plants or tree that you wish to be watered. Attach additional hoses or tape as needed.
Allow rain barrel to fill with water. When ready to use, turn on the spigot and let gravity pull the water to where you have directed it. Turn the spigot off when done or else you will lose additional water when it enters the barrel during the next rainfall.