Same as you do when using it on the level. Pull the lever back and push the button to lock. If parking on a hill, I'd also leave it in gear.
Yeah, it's me being a numpty. So here I am in a conventional car, approaching a T-junction on a hill. In a car with manual transmission, with my left hand I use the handbrake to progressively hold the car on the hill, while slipping the clutch and getting ready to pull way in first. It's that sequence I have failed to master with the fly-off, as it doesn't seem to have a progressive action.