How I did it: I got a three-year diary and every day wrote done some gratitudes (things that made me happy that day) and some acknowledgements (things I've done). In the beginning I'd write the things down as they happened, but later I started writing them last thing that night (or next morning, if I was really too tired to write).
Lessons & tips: Don't overthink. You are happy about something, that's enough.
Resources: Multiyear diary. It made it harder to skip days and after one year it was quite satisfying to read the entry of the previous year after writing the new entry. I chose a pretty diary and used a really nice fountain pen to add some luxury to the experience