Indeed, UV DTF Mini is a little more tricky compared with other Procolored printers, since it handles several types of films in a rather short time. To clearly solve this question, let’s first have a general draft of an UV DTF Mini side view with all the films involved marked out:
A film - A film is also called base film. A film is equipped with glue on it, so before it’s sent to get printed, there’s a protective film onto its glue side.
B film - B film is also called backing film. It’s a crystal film that will later be laminated with A film and transfer printings from A film.
Protective film - Protective film is referred in the Question as waste film, it in fact is the protective film of A film. This film must be peeled off before a printing task begins. 
Finished film - Finished film refers to the laminated film encompassing A and B films. 

Now let’s follow the red arrow marks to simulate a complete UV DTF Mini printing process and see where should the waste film go. The A film will be divided into two parts: protective film (waste film) rolls back to the roller at the upper back of the machine, the glue side of A film gets printed and cured by UV lights and then goes through the heating bar with B film, getting laminated and transferred, finally the finished film just exports from the film exit (or gets rewinded into the roll at the lower front of the printer if you’ve got an extra stand and rewinder).

So how to get the printer to print flat? The key point is to fine-tune the adjustment components according to the performance of your UV DTF Mini. There are adjustment components in 2 specific locations that we need to pay close attention to.
1) The ones near the orange heating bar. There are thumb screws at both ends of the heating bar that can be adjusted to load different pressure on the films. Key point is that these thumb screws have to maintain the same pressure in case the films deviate gradually.