Extended exhale breathing is the most evidence-supported technique. Inhale for four counts, exhale for six to eight counts. The longer exhale activates the vagus nerve, which directly signals the heart to slow and the body to relax. Five to ten rounds produces a measurable effect on heart rate variability.
Box breathing, four counts in and four counts hold and four counts out and four counts hold, is widely used in high-stress professions because it can be done silently and quickly. Consistency matters more than duration.