Patanjali and his commentators did little to clarify exactly how to practice pratyahara, or sense withdrawal. Thankfully, there are a few concrete techniques. Vayu pratyahara (wind withdrawal), also called prana pratyahara (life force withdrawal), is one technique that involves fixing your awareness and your breath sequentially on 18 vital points, called marmans, in your body.
To experience vayu pratyahara, take any comfortable seated yoga pose or your favorite reclining position, such as Savasana (Corpse Pose). Start with a simplified version of the practice by limiting yourself to just a dozen points: the big toes, ankles, midcalves, knees, midthighs, perineum, navel, heart center, throat well, middle of the eyebrows, forehead, and crown. You can add more later.
Consciously climb the 12-rung marman ladder to your crown. You can run through the points rapidly, spending just a breath or two at each one, or if you have the time and inclination, you can linger at each point for several breaths or longer. The former version of the practice challenges your ability to quickly and decisively direct both your awareness and breath; the latter challenges your ability to concentrate both awareness and breath over time.