Say what you actually thought.

If we’ve worked together — a full build, a quick fix, or an afternoon of me digging into your problem — a few sentences here is the most useful thing you could possibly do for a studio this new. Two honest lines are worth more than a paragraph of praise; the specific bit is what a stranger reads.

And if it wasn’t all good, say that too. I’d rather know than be flattered, and I’m not going to argue with you about it.