The first step to the block start is to determine which leg is the power leg and which leg is your speed leg. This is important because the power leg will be placed in the front block and used to propel the body. 

There are several ways to determine which leg is the power leg, but I like to use two pretty common methods. The first method is to stand tall with your arms by your side, on command, bring your arms across your chest as fast as possible. The arm that hits the chest first is usually your speed side and the arm landing on top of the other is usually your power side. The other method is to stand tall with your feet slightly apart and have someone come up from behind and give you a push forward. The leg you use to break your fall or to catch yourself is usually your power leg.