Research on habit formation found habits take an average of 66 days to become automatic, with a range depending on complexity. 21 days is a myth that sets people up for disappointment.
Attach the habit to something you already do every day without fail — brushing teeth, making coffee, sitting at your desk. The trigger must be 100% reliable or the stack won't hold.
No, missing one day is normal and doesn't break a habit. What matters is never missing two days in a row, which is when a slip turns into an actual pattern change.