You need cybernetics (as in the feedback loop, the habit that monitors the process of adding habits). Meditate and/or journal. Therapy is also great. There are tracking apps that may help. Some folks really like habitica/habit rpg.
You also need operant conditioning: you need a stimulus/trigger, and you need a reward. Could be as simple as letting yourself have a piece of candy.
Anything that enhances neuroplasticity helps: exercise, learning, eat/sleep right, novelty, adhd meds if that's something you need, psychedelics can help if used carefully.
I'm hardly any good at it myself but it's been some progress.