This may seem like a no-brainer, but you won’t believe the difference it can make in your final baked product if your egg whites are folded in correctly.
Firstly finish with the base mixture into which the whites have to be added. Only then whisk the egg whites. You get soft peak, which kind of flops over when you lift out the whisk. Stiff peak holds its shape perfectly, and this is usually the stage which most recipes call for. Hard peak is when the egg whites are whisked beyond stiff peak and you almost get a broken effect – few recipes will ever require these egg whites.
Once you have your whisked egg whites, place about a third of it into the bowl with your base mixture. Using a metal spoon, stir in the whites to lighten the base mixture. This can be done quite vigorously then add the remaining egg white and gently fold it into the mixture using a “cutting” motion the a metal spoon. Don’t rush the process or you will lose volume.
Stop once all the egg white is incorporated.