Recipes such as soufflé, mousse and angel food cake ask you to fold egg whites or whipped cream into a batter, but don't begin to tell you how. If your batter folding skills aren't up to scratch, Emma Christensen at The Kitchn has put together a great tutorial.
Doing this properly will take some practice, but basically you need to use a large, flexible spatula to slice the egg whites or whipped cream down the middle, scoop and fold one half over the other and into the batter, turn the bowl 90 degrees, and repeat. The key is to go slow, and not to overdo it. It's time to stop once you no longer see liquid batter dripping off of the spatula. Fold too much, and you'll deflate the batter and ruin the fluffiness of the finished product.
For some photos, videos, and a full walkthrough of the process, be sure to check out the source link.