A quick thank you is often enough, but a specific note can tell someone what their help made possible. It also lets them know that the material or time they provided was useful.
Name the action you appreciated: checking the figures, bringing the document or making time to explain a process. Say what you were able to do with it, using only what actually happened.
Keep the scale natural
A small favour does not need an extravagant description. A few clear sentences can carry the point. If the help involved a shared project, acknowledge the other contributions fairly as well.
Choose a channel that fits the relationship and the privacy of the work. Ask before making private help into a public endorsement. You can thank someone even when the eventual outcome was disappointing, provided you distinguish the help from the result. The note is about the contribution you received, not a claim that the person caused every later success.
