Somewhere there is a person you never properly thanked. Write the letter to them, by name: what they did, what it cost them, and what you gained from it in your life. Sending it is optional.
Writing it is a gift you give yourself before you give it to anyone else. You might choose someone you thought about during the day, or who sent you a message, or whose news you saw on one of the social networks - and then it becomes part of your positive writing about the day.
A gratitude journal counts blessings; a gratitude letter faces a human being.
Send it or don't send it; that is a separate decision, for another day. The research says the writing itself is what does the work, whether or not the letter ever arrives.
For instance
Mrs Doyle - you stayed after school for a fortnight in Year 9 to get me through the maths, and only years later did I work out you had a bus and a child of your own waiting. I teach now. I stay behind for one student. I know exactly what it cost you.
Uncle Ray: the year Dad was ill you turned up every Sunday with the shopping and never once made it a favour. I have been living off that quietness ever since, and I have never said thank you out loud.
Choose one person you never properly thanked. Write them a letter - what they did, what it cost them at the time, and what it made possible in your life. Whether you send it is up to you.