Begin your exercise routine by warming up on some long tones. You should warm up your voice before any exercise, to minimize the risk of hurting your voice.
Position yourself in such a way that you are using proper posture, and able to provide good breath support. You should be sitting, or standing, straight with your shoulders relaxed, while taking air into your diaphragm. Avoid the tendency to take shallow breaths that end up in your chest.
Sing a pitch higher in your falsetto range. Hum the interval of a fifth, including all of the notes in between, and then ascend back to the starting pitch. For instance, sing the notes C - B - A - G - F, and then ascend to C again.
Continue singing the fifth exercise five times each time, moving down by half step. For instance, if you started on a C, move down to a B, then Bb and so on.