If you’ve never done it before, the idea of working with a pastry bag to decorate a cake with icing or to pipe out pâte à choux to make éclairs might seem a little daunting. But it’s actually quite fun, especially once you get the hang of loading the filling into the bag. Here’s how to make this potentially messy step neat and easy: First, attach the piping tip, using a plastic coupler if need be (consult the instructions that came with your tips). Fold the top of the bag into a wide cuff, and hold the bag under this cuff.
1. Using a long spatula, transfer the filling into the bag. With the hand holding the bag, pinch the spatula as you pull it out of the bag to slide the filling off the spatula. Once the bag is half full—don’t fill it more than halfway or it will be hard to pipe—unfold the cuff, lay the bag on the counter, and use a bench knife or the side of your hand to force the filling into the tip of the bag.
2. Twist the bag closed and, before you begin piping, squeeze a little of the filling into a small bowl to force out any air tripped near the tip.