Click your mouse or tap in the filling area to release the balls. Make sure you don't have too much of a flow happening or you will knock your balls out of play! Once your glass is getting full make sure you slow down to not overflow. The objective is to fill the glass to the dotted line without overflowing.
On your mobile device tap your finger in the touching area to release balls into play. If you're on a desktop device click your mouse in the same area.