The pulse sensor can be attached to either the ear lobe or fingertip. In my experience, attaching the sensor to the fingertip with the included velcro strip provides the most reliable reading.
Once the sensor is secured, connect the Circuit Playground board to power via the USB or battery connector and you should see the two LEDs near the USB jack begin to flash in time with your pulse.
If the LEDs don't light up, try readjusting the sensor until you have a secure fit.
Breath Pacer: Eight of the neopixel LEDs on the Circuit Playground are used to display a breath pacer which is helpful for ideal timing of inhale/exhale cycles. Breath in as the LEDs pulse toward one side, and breath out as they move back to the other side.
Pulse/Relaxation: The remaining two LEDs flash with each pulse detected and their color indicates a basic HRV rating which roughly translates to the user's level of relaxation:
red = low relaxation
blue = moderate relaxation
green = high relaxation
Sit in a comfortable position and use the breath pacer to guide your breathing - breath in as it moves toward one side, and begin to breath out when it changes direction.
As you continue to breathe at this slow deliberate rate, you should notice the pulse LEDs change color to indicate smoother changes in heart rate variability and hopefully help you achieve a more deeply relaxed state.
Though the relaxation indicator is helpful, the most useful functionality here is actually the breath pacer. One of the most common techniques for achieving a meditative state is focussed breathing. Paying conscious attention to your own breathing is a simple but highly effective way to increase mindfulness.