Filling a piping bag can be a bit tricky, but with a simple technique, you can avoid a messy situation. First, insert your desired piping tip into the bag and push it down until it is secure. Then, fold the top of the bag over your hand, creating a cuff. Fill the bag about two-thirds full with your prepared frosting, while making sure not to overfill it. Finally, twist the top of the bag and gently squeeze the frosting downward, removing any excess air.