Take item A and replace it with three subitems, A1, A2, and A3. Do the same for B and C. See Figure 2 below.
Replace A1 with A1a, A1b, and A1c. Yadda yadda for B and C. See Figure 3. It's starting to look kinda snowflakey, which is how the model got its name.
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You're probably not going to have exactly three subitems for every item, but you see the basic idea. In software they call this top-down design. You start with something simple and break it into pieces. Then you break the pieces into pieces. Each step is simple, so you don't get overwhelmed. In the end you have something complex yet manageable.
Please please take note. This model may not work for you. The best advice I can give you for outlining is to try as many models as you can find. What works for one person may not work for another. What works for one book may not work for the next. You may need to combine elements of different models. This is not an exact science.