Sitting or lying comfortably...
Breathe in for a count of four
Hold the breath for a count of four
Breathe out for a count of four
Hold the "empty" breath for a count of four.
Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat.
When you stop, feel your heartbeat, pause and take a moment to review your mental state.
What did that do for you?