While it is possible to start flying by clicking on the fly button at the bottom of the screen, it is actually better to initiate it using the PgUp key. Just hitting the PgUp key once, will cause your avatar to jump up and down. Keeping it down will convince the avatar to actually start flying. To attain greater heights, just use the PgUp key for as many times as needed. In the event that you want to fly at a lower hight, use the PgDn key. As from use of the PageUp and PageDown keys, flying around is very much the same as walking around.