Very similar to meditation. The best meditation advice I got was to just sit down, focus on doing nothing and be OK with being bored. Not some breathing techniques, how to focus your mind, posture etc. That comes later, very naturally, after you get curious about improvements.
Same goes for writing down thoughts. Just take a blank A4 paper from the printer and write away. Not sure what to write? Write about that, the fact that you're not sure what to write. Throw the paper away if you don't like the thoughts you see, or at least do it with that mindset. It's not about writing something novel, just as meditating is not about reaching some enlightenment