If you are having trouble breaking a big topic down into smaller ones, you might want to try clustering. This is a tactic in which you write down a very broad topic or idea and then make a concept map, in which you diagram smaller ideas or categories (clusters) that relate to the central topic.
Say you are writing a paper that teaches your classmates how to perform a task. You have one page to provide detailed instructions about an activity. If you have chosen "gardening" as that activity, you will not be able to give an adequate description in the space provided. There's simply too much information. You have to choose a smaller task associated with gardening. The question is, which one?
All the things you linked to "gardening" are smaller tasks you could describe. You can even break them down into further levels of detail. For example, the subcategory of "researching and purchasing plants and seeds" can be broken down into separate bubbles for research and purchasing. The purchasing bubble could be broken down into a) where to purchase plants, b) when to purchase plants, c) how much to pay for different plants, and so on, until you reach the right level of specificity. In this way, you can break your general topic down from "explaining gardening" to something like "explaining how to purchase a sunflower plant."
The goal of clustering, much like freewriting, is to come up with lots of different possibilities. Then you can choose which ones you think are best suited for your assignment. Make sure that you don't censor yourself when you're making a concept map: add anything you feel is related to your topic and let it flow!