hot water (not boiling, but say tap hot), water quality suitable for rinsing and drying film afterwards, wet film and 2 slides of glass (same cheap photo frame glass), place film between two slides of glass, so that it is sandwiched flat, place the entire thing in the tap hot water for about a minute. Take it out, hang it up to dry.
You can also do a similar thing but dry, have the film in it's plastic sleeve, heat up with a hair dryer on both sides, then press it flat against a cool surface.