It's only hard to spell if you have no clue about french ;-)

Here's a memory hook:

- "ou" because that's french for "oo", as in: douche.

- "l" since it comes from "roule" or "rouler", as in: joule, boule (there is no "ll" between "ou" and "e" in french, ever).

- "tt" since it's a female noun, as in: cassette, baguette, couchette.