I would pose the question - “what would 100% ensure that you do it?”
A gun to your head ala Fight Club with a bullet as a consequence would.
So all you have to do is design a system where the consequences of not doing it are greater than doing it.
1. State ONE habit per month you’re looking to change publicly
2. Get your friend to state there’s
3. Loser buys lunch
4. Track it publicly on habitshare so they get notifications when u succeed/fail
(had extreme behaviour change via this)