Cultivating gratitude can be as simple as expressing gratitude. This small practice is described in greater detail on the mindful.org website.
Start by observing. Notice the times when you say “thank you.” How habitual are they? Are you thanking the person for something specific, or just being polite?
Pick one interaction a day. When your instinct to say “thank you” arises, stop for a moment and take note. Can you name exactly what you feel grateful for, even beyond the gesture that’s been extended? Then say thank you.