For a single application on an area like your lower back or both shoulders, a pea to a hazelnut-sized amount of gel is a good starting point. The key is that a little goes a long way because the gel spreads easily. You can always apply a bit more if needed after a minute, once you feel the initial sensation.
Apply it to clean, dry skin and use your fingers or palm to spread it evenly in a thin layer over the tense area before beginning to massage. The goal is to have enough product to allow your hand to glide smoothly without excessive friction. Because the formula contains menthol, camphor, and other active ingredients, always wash your hands thoroughly with soap and water after use to avoid accidentally transferring it to your eyes or other sensitive areas.
Avoid using it on broken, sunburned, or otherwise irritated skin, and do not apply near mucous membranes (eyes, nose, mouth). If you are very reactive to aromatic oils or strongly warming products, it's wise to patch-test a tiny amount on your inner forearm first.