Blowing

blowing-clock

Slowly, like breathing out normally. Blow from the back of the mouth not the front. This will produce a more solid, rounded sound. Blowing from the front, as one does when whistling, will produce a more harsh, hollow sound.

You should also practice by timing how long you can hold one note for. Try to keep your breath at a constant rate to keep the note even and controlled. You will need slightly more breath for the higher notes. Presumably there’s a physic explanation for this but I don’t know what it is.