Way 1 · best default
You've got this
Short and confident. You're telling them they're ready, right before they start.
Examples
Big interview tomorrow? You've got this.
Deep breath. You've got this, you've practised it a hundred times.
In a conversation
A: I'm on in five minutes and my hands are shaking.
B: You've got this. Just pretend it's us in the front row.
Common mistake
Not: You can do it well!
Say: You've got this!
"Do it well" sounds like a teacher's instruction. "You've got this" sounds like a friend who believes in you.