Spray them clean and test the downspout to make sure there are no clogs.
Run water down the downspout to make sure it is clear.
To do so it's best to start at the end opposite the downspout and aim the hose toward it.
A good kit will contain a complete set of instructions on how to install a rain barrel to the downspout of your rooftop rain gutter.
Most rain barrel kits provide you with a diverter or adapter so you can connect directly to the downspout of your rooftop rain gutter system.