Saying it now
Once you've seen the fine, say it. This is the part that reaches the other person, and it's the only part that does.
To the person. Not to the room, not to a third party later, not in your head. To the one who did it, looking at them.
Now. At the time, in the moment you see it. Not saved up for a card or an anniversary or the moment they threaten to leave. The thanks that lands is the one that comes with the thing it's about still in view. "Thanks for laying the table" while the table is laid. A thanks delivered a week later is a different thing, still worth doing, but it doesn't correct the record in the moment the record is being made, and the moment is where the commentary lives.
Specific. Name the thing. "Thanks for sorting the post." "The section on the north region is really clear." "Eighteen out of twenty, and you got the fractions ones." Specific thanks can't be faked, because you can only say it if you saw the thing, and the seeing is what the other person is responding to. General thanks, "you're great", "thanks for everything", asks them to take your word for it. Specific thanks shows them the evidence.
Short. One sentence, two at most. A speech about how much you value them turns thanks into an occasion, and occasions are awkward. A sentence isn't. The awkwardness you're forecasting comes mostly from imagining a speech. Don't give one.
No "but". "Thanks for laying the table, but the pan's still there" is not a thanks. It's a fault with a run-up. The other person hears the pan and learns to brace when they hear the word thanks. If the pan needs mentioning, mention it later, in a separate sentence, ten minutes on. The two lists stay separate in speech as well as in your head.
No "finally". "Oh, you did the table, finally" is a fault dressed as a fine. So is "about time". So is "see, you can do it". If a fine is worth saying, it's worth saying without the sting, and if you can't say it without the sting, say nothing and come back to it when you can.
No comparison. "Thanks for the table, that's more than your brother ever does" makes the thanks about someone else, and the person in front of you learns that they're being ranked. Just the thing, just them.
Out loud. Thinking it doesn't cross the room. Lambert's experiment found that thinking grateful thoughts about a friend did less for the thinker than saying them to the friend did, and for the friend it did nothing at all, because the friend never heard. If you can't manage out loud yet, a text is a start. "Just saw the post is sorted, thanks." It's a poor second to a face, but it crosses the room.
The first week. It will feel strange to you. Chapter 9 said why, and the evidence says the strangeness is yours and stays on your side of the room. The other person may look surprised, and the surprise is the point. They didn't know. If they say "what's brought this on?", the honest answer is fine. "I noticed I only ever mention what's wrong, and I'm trying to mention the rest." Most people, told that, will help you.