Flyballs are easily simplified into a few steps. First off, when you see a ball hit in the air to you, your first move is to take one step backwards with your throwing-side foot to gain a ready position. Once you judge the ball's path, start moving towards where it is going to land. Don't attempt to get directly under the ball, because a lot of the time the ball will carry slightly further than you expect. Instead, position yourself slightly behind where the ball looks like it is going to land, and as it reaches you, start gradually moving into it. In a situation where a throw needs to be made quickly, you can use this slightly set back positioning to make a crow hop. See the video below for a visual walkthrough.