How to flatten a film roll

The best thing to do is to roll it for a while. When rewinding, pay attention to whether the negative is dry or not, if it is dry, it will be in danger of breaking. It's best not to sandwich it in a book, as it will make the negative too dry and break, and the book will have bleach on it, which will affect the negative. It is better to put the negatives into the bag after flattening and save them, which is usually provided by the printing department free of charge.