Choosing the correct vacuum pouch is essential. Make sure they are from a reputable source and are specified as cook-in or boil-in-the-bag pouches. It is good to have three sizes in the cupboard: small, medium and large. When filling a vacuum pouch fold the top few centimetres of the pouch out, as this leaves the top of the pouch where it will be sealed nice and clean when filled with food or marinades. Never fill a pouch more than two-thirds, as it will be extremely awkward to manage when sealing. The seal that is formed by the vacuum-sealer is simply a melting of the two sides of the pouch together. If the seal is not perfectly flat or if it has a fold in it, it will not remain airtight during cooking. If this happens, transfer the food to a new pouch and try again.