You will find the direct answer in this video: https://www.health.harvard.edu/video/positive-psychology/what-it-takes-to-be-happy-Q5.htm.
To summarize there are 5 broad paths being discussed:
1. Use our strenghts/virtues: our sense of wisdom, justice, curiousity & compassion
2. Gratitude: appreciating what you have and expressing that appreciation to yourself and to others.
3. Savoring: tasting the moments, example: taking the time to smell the moments not always rushing for something.
4. Flow (see mihaly Csikszentmihalyi)
5. Live a life thats meaningful and most of the time it means doing things for others. Authors note: this doesn't mean it has to be exclusively for other people.