The short answer is that you don't!

However, it isn't as uncontrolled as that might seem.

The pilot gets full aviation weather reports various sources, which may include governmet or private services. Prior to flight the pilot releases small helium balloons to see the wind direction and speed. Often there are layers of wind that are going in different directions or moving at different speeds.

After anaylizing the weather reports and the actual conditions observed, the pilot will make a decision to fly or not, as well as selecting a launch site appropriate for that flight. since the balloon floats with the wind, the pliot knows which direction the balloon will travel so the launch site is selected which will allow the balloon to fly into an area with appropriate landing sites.

Once airborne, the pilot can move the balloon up and down to utilize the varying directions of wind to change course. Some days the change in direction is quite significant, while at other times there may be little or no change in direction at any altitude. All these factors are taken into account when the pilot determines if the flight will take place or not.

There are even days when there may not be enough wind movement above the earth to fly at all. If there is little or no wind, or if the wind direction is constantly changing, the pilot may decide it isn't safe to fly. If the balloon just sits overhead with little movement, it may be very difficult for the pilot to find a suitable landing spot.