In your head, it goes fine. In the room, you hear yourself say “maybe I wasn't clear enough.”
Nobody avoids these conversations because they don't know what to say. They avoid them because of what happens on the fourth minute, when the other person is upset and the standard you set starts sounding negotiable.
You can't practise that alone. You can't practise it on a colleague either, because they'll be kind. So the first time you handle the pushback is the time it counts.
This gives you the fourth minute, as many times as you need it, before Thursday.