Place one gel pad flat in the microwave, so not folded or creased. Heat for 20 seconds at 385 W or 25 seconds at 600 W. Check the temperature: the pad should be warm, not hot. Not warm enough yet? Then heat for an additional 5 seconds. Dry the pad and apply it in a cover to the painful area.
Heating in water is also possible. Bring water to a boil and remove the pan from the heat source. Then immerse the pad for 2 to 3 minutes, for example with tongs. Do not leave the pad unattended and keep it away from the bottom of the pan.