The watch should fit comfortably and snugly - not loose but not too tight. It should sit flat on the top side of your wrist, 2 inches away from where your wrist and hand meet. It’s important that the watch make contact with the top of your wrist at all times, so it can get a good reading.
When the watch fits properly, the ‘Heart’ icon will appear on the watch face to indicate the watch is getting a good heart reading. If you don’t see this icon, you may need to readjust the watch.