A useful addition to "say it once": say what would change your mind. "I think step 2 fails because X; if this small test passes, withdraw that objection." Now the other person has a way to resolve the disagreement without accepting your authority.

I would also separate "your premise is false" from "your plan can reach your goal, but I would choose a different goal." The second disagreement can hide inside a lot of apparently technical advice.

If I cannot name a result that would satisfy my objection, I should check whether I am defending a preference with a lab coat on.