Availability of this feature depends on your vehicle application as well as having it activated within your tune.
Method of Operation
Make sure the vehicle is up to temperature (coolant, engine oil, and transmission temperature)
Ensure the environment in which you are using it is safe and proper according to local laws and regulations.
As you want to start decelerating put your foot on the brake.
When you want to shift gears down, leaving your foot on the brake, press the clutch all the way in.
You should see RPM get bumped upward allowing you to easily change gears and take your foot off the clutch.
At that point you can continue to decelerate with your foot on the brake, and repeat steps 4 and 5 until at a complete stop.
It's recommended to push the clutch in fairly rapidly. Going slowly can cause the throttle blip to be activated while the clutch is still partially engaged which can potentially cause damage to the drivetrain of your vehicle.
Use of this feature can be hard on the drivetrain and should be done at the users risk.