If the product is real TopFlite MonoKote, start with about 275 degrees F and adjust temp up till the film adhesive gets tacky. If film melts, you went too hot and turn down the heat.
See link below.
MonoKote Tips Page - Click HereOther types/brands of iron on covering may require different temps for best results. Start with low temp and keep turning up the heat till the adhesive on film will activate and get tacky.
If for some reason the film adhesive will no longer activate properly and not stick good enough, I would use some medium CA applied between film and where you want it attached and hold film in place with some non stick plastic under finger to keep from getting glued to any CA that will creep from the edge of film.
Good plastic to use is the peel off backing from MonoKote or even something like Saran Wrap.
Also, there is a MonoKote/TopFlite brand liquid product (Trim Solvent) available to activate the film adhesive without any heat needed.