Breath-based mindfulness practices are some of my favorites - you don't need any extra tools or to be in a particular location. The breath is always with us and a powerful tool for regulating your nervous system. Slowing down your breath activates the parasympathetic nervous system ("rest and digest"), which can help put the brakes on your fight or flight response.
How to Practice Alternate Nostril Breathing
Place your thumb on your right nostril and ring finger over your left nostril.
INHALE: Close your right nostril and breath in through your left nostril for a 4 count.
PAUSE: Close your left nostril (both nostrils closed) and pause.
EXHALE: Open your right nostril and breath out through your right nostril for a 6 count.
INHALE: Breath in through your right nostril for a 4 count.
PAUSE: Close your right nostril (both nostrils closed) and pause.
EXHALE: Open your left nostril and breath out through your left nostril for a 6 count.
Repeat.