If you bought a flexible downspout extension chances are you're trying to redirect rainwater from making a puddle at the base of your foundation. With a flexible downspout extension you can redirect rainwater around bushes and cement to have it flow into your garden, to a flowerbed or just away from your house.
Find the end of the downspout extension that fits on the downspout. Most flexible downspout extensions have smooth ends on either side of the accordion-pleated center section. Hold the ends up to your downspout to see which end matches the downspout size.
Stretch out the downspout extension until it is fully extended. Unkink and straighten before installation to avoid breaking the extension or pulling it off the downspout.
Cut the end off the extension that will not be connecting to the downspout. Use utility scissors or a box cutter to cut along the line that's factory marked on the extension. Once the downspout extension is cut it is ready to connect.
Take the side that fits around the drainpipe and slip it around the drainpipe. It should slide right on. Bend the downspout extension where it hits the grass to flow into your garden or where ever you want. It should fit snugly, without any need to clip it on further.