My general rules of thumb:

1. Be yourself. Speak the way you normally would.

2. If someone asks for advice, then offer it. Answer the question. But don't take the opportunity to unload everything else that's on your mind.

3. If no one asks, then wait for the end of combat. If it's a wipe, ask whether anyone has questions, or phrase your suggestion for preventing the wipe as a question ("Maybe we should...?"). But if it's the first wipe and no one speaks up, maybe just give it another go. Sometimes people learn; sometimes they'll get more frustrated and become more open to advice.

4. Try to phrase things in a way that makes it clear everyone has a part to play. For example, not, "Healer, take the stack marker mid." Instead, "That might be easier if we all stacked in the center."