Lean slightly forward. Blow your nose first to remove blood clots. Breathe through your mouth.
Press without interruption the nose, just above the turbinates (the soft part of the nose), for five to ten minutes tightly with thumb and forefinger. The pressure forms a clot on the damaged blood vessel causing the bleeding stops almost certain.
Do not:
keep the head back, or plug cotton or compresses in the bleeding nostril. That leads the blood to the throat.