Free Writing
There is no one way to free write, but there are a few rules:
Set a Time
Don't Stop Writing
Write ANYTHING
Edit NOTHING
Here is my recommended process. Always remember that writing is an expression of your self, like dancing or singing, so no two people will free write the same.
Prepare - Take 60-120 seconds to setup your computer, table, etc. Get tea ready, plan your time, shut or open windows, etc. Make sure you have no reason to stop in the middle of your free write. This is important.
Write - Whether it is 60 seconds, 5 minutes, or 30 minutes, write write write within the time you set. The only rule is that you CANNOT stop. Sometimes, it takes ten pages of bad writing to get 2 sentences you love. Anne Lamott has an incredible book on writing called Bird by Bird. If you can, check out the chapter "Shitty First Drafts" (Link here).
Reflect - When time is up, review your writing and highlight sections that have potential for use. Look for ideas, not phrasing, syntax, refinement, or etc. You're not trying to arrive, you're trying to get going. Transfer what you can into an outline and move on!