How to Expand like a Pro in a NutshellBased both on internet research and personal experience.
You order your expander (="Active Plate" for the upper jaw) including the impression kit from bracesshop.de. They send you the kit first, you make your impressions and send it back to them. Within two weeks they send you your custom made expander. Voilą, it fits perfectly.
I expand approximately one turn (=0.25 mm) every other day. The max amount of expansion for this expander is 8.5 mm.
Thanks to the fact that I have gone through traditional orthodontic treatment, I have a very good sense of what pressure and pain is "normal" and what could possibly be harmful for my teeth and jaw. If I was able to wear the expander during the whole day, I think I could expand even faster, maybe one turn every day.
Sources:
Mike Mew - Youtube video "Is Palatal Expansion Possible In Adults By Dr Mike Mew"
https://www.realself.com/question/palate-expander-for-24-year-work - answer given by Brad Lockhart