One more note about self-editing the content of your middle:
As you work to write details, descriptions, and anecdotes in your middle that support your main idea or develop your main story problem, plan to include those that are most important or carry the strongest weight. If you’re struggling to decide which to include and which to cut, an exercise that helps me is to step away from writing for a moment. Talk with people (especially kids!) about your main idea instead.
Call a friend and say, “Hey, I’ve been reading the most interesting things about William Penn. Remember him? He’s the guy who started Pennsylvania. Did you know…?”
As you talk with various people about your topic, make a mental note of which details keep rising to the surface like cream in a gallon of whole milk. Listen to yourself and note what you’re passionate about sharing. Make a mental note of which details your listeners are eager to hear more about. These are the details you want to include in your manuscript’s middle and develop into anecdotes and scenes. Just be sure to present them in a way so that they work to support or develop your main story problem or main idea.