A popular technique for writing fiction novels is known as the Snowflake Method. It’s called “the snowflake method” because it is like creating an intricate snowflake, you start out with a basic shape and then refine each of the parts further down until the result is quite intricate:
While the instructions for the Snowflake Method are really for novels (there’s a lot of talk about characters), it’s easy enough to modify it to use it to plan and write your non-fiction ebook instead. I’ve used this method to write an ebook with three other people across three continents in about three weeks and it worked very well. I use it regularly in most of my non-fiction writing (it doesn’t have be just for ebooks) and highly recommend it.