Arm out straight, palm down — use the other hand to gently pull the fingers back and feel the top of the forearm. Then flip palm up and gently pull the fingers back the other way for the underside. Light and brief, both sides — this is a primer, not the long hold you'll get later in a finisher.
Showing: Kneeling Forearm Stretch — closest verified demo. Shows the free-exercise-db's own 'Kneeling Forearm Stretch' — a kneeling, palms-flat-on-the-floor variant that stretches the same wrist-flexor/forearm group by leaning back through bodyweight rather than standing with the opposite hand pulling the fingers back. It depicts the flexor (palm-down) half only; the extensor (palm-up) half prescribed alongside it isn't shown in this photo pair.