Put on protective eyewear. Use a drill with a screwdriver attachment or an electric screwdriver to remove the screws holding the downspout to the wall. Take off all the metal downspout until the point where it first bends and touches the wall.
Place two cinder blocks underneath the downspout to form a flat platform. If the ground underneath the downspout isn't level, dig out the soil until you can form a flat surface with the top of the blocks.
Place the rain barrel on top of the cinder block platform. Orient the spout so that it aims away from the building.
Attach a downspout redirector to the bottom of the downspout. This tool is basically a rectangular funnel that diverts rain water from the gutters into flexible hose.
Attach a flexible plastic hose to the bottom of the downspout redirector. Use the hardware enclosed with the redirector package, or hold them all together with layers of waterproof duct tape.
Position the end of the flexible hose over the rain barrel. Rain will pour down the spout and into your barrel, ready for you to use to water your plants.