One advantage I had with this assignment was an old video I had of the setting I wanted to describe. That helped a great deal.
And while description is the main focus of this assignment, it helps to have some semblance of a story tucked away within. Set some kind of minor story scene at the beginning to hook your instructor, and close with some sort of resolution or continuation of that scene.
For instance, if you're writing about your clubhouse, I'd recommend starting out with a scene involving a sleepover or secret meeting with your friends or something like that. Maybe even when you're actually building the clubhouse, that would help too!
But don't get too involved in the scene. This IS a description assignment, after all.
My last bit of advice is this: Describe the scene as your first-person, younger self. Unearthing your buried childhood memories of your chosen setting is one thing. But if you really want the scene to come alive, you should describe it in your child-self voice. Use the kind of language and style you would've commonly used as a 10 year old, or an 8 year old, or whatever.
Metaphors and similes help, too!
Hope this was useful to you!