A gratitude letter is simple in concept: write to someone who helped you, describing specifically what they did and how it mattered, then, where possible, deliver it in person. The specificity is what separates it from an ordinary thank you, since it forces the writer to recall a real moment rather than offer a general compliment.
The effect tends to surprise both sides. The writer often underestimates how meaningful the letter will be to the recipient, while the recipient is frequently more moved than expected, partly because being thanked so precisely for something they may have half forgotten doing is rare in ordinary life.
It does not need to be reserved for dramatic gestures. A teacher, a former colleague, or a neighbour who helped during a hard week are all reasonable subjects, and the letter tends to work just as well, sometimes better, when the gesture it describes was small.