Drains can get clogged from a multitude of sources. Hair, grease, mould and other debris collect over time to form a mass that impedes water flow. Outdoor drains have the added problem of leaves, dirt and additional garbage or plant debris contributing to clogs that need to be cleared. A pipe snake is a metal cable that is designed to be forced down a drain until it penetrates a clog, then yanked back out to clear the clog from the drain. Snaking an outside drain may require multiple attempts to do the job thoroughly.
Remove the drain cover. Some outdoor drain covers have openings in them large enough to accommodate a thin drain snake, in which case you can leave the cover on if removing it is problematic. However, you won't be able to reach into the drain to pull out any clog that remains.
Push the end of the pipe snake down into the drain.
Extend the cable from the reel that it is wound on and force it as far down into the drain and pipe as you can. Wiggle and twist the cable if it gets stuck to try to force it through or around whatever is blocking it.
Yank the snake cable out once you've pushed it down as far as it will go into the drain. The force of the cable being removed will dislodge clogs.
Repeat the process several times to increase the odds of removing any major clogs.
Reach into the drain to manually remove any remains of the clog that the snake has brought back up. Wear thick rubber gloves.