Commence with whatever is easiest and most interesting to you. This gets you into the flow and begins to release your creative juices. It's a good idea to write the Introduction early since this sets the stage for the whole book. An Introduction tells the scope of the work and details how people will benefit from reading it. It also keeps you focused on the target.
Lean sentences are the most inviting to read.
So are concise paragraphs. Use short, pithy words. It's a place to communicate. Such flavoring gives your message zest. One approach some of our clients like is to dictate their thoughts, have the tape transcribed, then partially clean up the manuscript pages before sending them to us.