If you make a balls-up of something, you do it very badly and make a lot of mistakes. He's made a real balls-up of this. = cock-up. something that has been done very badly or not successfully
If you balls up a task or activity, you do it very badly, making a lot of mistakes. You have single-handedly ballsed up the best opportunity we've had! I have no intention of letting you balls it up. = cock up