Pick the gutters that you wish to place rain barrels under, then choose what size rain barrels you will use. Dig up with a shovel and remove any vegetation under where the gutter is draining water. The actual amount of area you will have to clear depends on the size of the rain barrel. Clear any vegetation so that the rain barrel will not sit on it, making it unlevel.
Measure the height of the rain barrel and add 4 inches to this measurement. Measure this height on the gutter running down the side of the building, starting at the ground underneath it and going up. Mark the gutter at the required measurement with the marker.
Cut the gutter with a hacksaw horizontally along the line you marked. Then place the rain barrel underneath the gutter so that the water will drain into it.