First cut a piece of film to the right size. Then seal one of the open sides so that you have a bag of the desired size. Place the bag with the open side in the vacuum sealer and vacuum/seal.
TIP: Our original films and rolls are extremely stable and tear-resistant. They can even be cleaned in the dishwasher after use and then reused. It is therefore better to cut the bags more generously in size so that you can use them more often.