In order to ensure that you are working in a safe environment, disconnect the garage sensors from the power source.
Loosen the screws holding the sensors in place just enough so that the sensor will shift position, either upwards or downwards, when you apply a small amount of force.
Using a tape measure, adjust the height of the sensors above ground level until both sensors are exactly 6 inches off the ground. This is maximum height limit for normal operation of the sensors. Above 6 inches of height off the ground, the garage sensors may not work properly.
Now you need to simulate the infrared beam which is emitted by the transmitting sensor and is received by the receiving sensor. Run a piece of string from one sensor to the other and tie it firmly. Make sure that you tie the string at the same point for both sensors. For instance, if you tied the string to the bottom of one sensor, you have to do the same for the other sensor as well.
Using a level balance, make sure that the string is perfectly level with the ground. Adjust the height of the sensors if that is not the case. Keep making slight adjustments until the string attaching the sensors is perfectly levelled. Once this is accomplished, firmly tighten the screws on both sensors with a screw driver.
Untie the string and plug the sensors back into the power source. To test the garage sensors, roll a basketball between the sensors.