The first draft is your draft. Nobody else should see. In fact, showing a first draft to somebody is an offense punishable by death in some cultures. Seriously, don't let anybody read your first draft.
The biggest benefit of this is that gives you a lot of freedom. Freedom to make mistakes that only you will see. The freedom of knowing that those mistakes will be kept a secret between you and your writing desk.
If you see a mistake while typing the first draft, just ignore it. The dangers of going back and fixing a mistake is that it will pull you out of the story and kill your momentum. You can easily lose MINUTES of writing productivity to correct a two second issue that you could save for later. Yes, I'm encouraging selective procrastination. Trust me, ignore the mistake, and keep moving forward.
The second draft is a big step in the writing process, however, you will never get to that second draft of a completed book if you are so bogged down trying to make the first draft possible that your book never gets written. Write the first draft and give yourself something you can fix and improve afterward.
To be continued...