Use your fingers to press your nostrils. Don’t block the passages completely; just reduce their size by narrowing the openings of your nose. Relax, and then slow down your breathing, imagine you are breathing through narrow straws. Give yourself some time to adjust to this reduced breathing. Feel free to modify the size of the air openings; you can make this exercise easy or difficult as you like.
The main goal of the Buteyko Breathing Method is to reduce air consumption. This is exactly what you are doing by following this exercise, which is good for everyone but particularly for people with seemingly forceful breathing.