It all depends on the type of tire gauge you're using, but in general there should be some sort of readable dial or spring-loaded stick which will move according to the amount of pressure in the tire. The numbers on the dial or stick should refer to pounds of air pressure. If a tire's recommended air pressure is 35 pounds, for instance, you would apply the tire gauge over the valve stem and look at the number on the dial or stick. If it points or stops at 20, you'll want to take the tire to an air compressor and pump in some additional air. The compressor itself may have a tire gauge attachment, or you can use the original one. Once the gauge reads 35 pounds, cap off the valve stem. If it reads higher, release some air by depressing the small valve stem and take another measurement.