Hold the iron face up, level. Apply enough clean water to the face to bead, but not roll off. Adjust the switch to boil the water when it clicks off, and is not boiling when it clicks on (yes, you need to reapply the water a few times)
Mark this location as 100C/212F It is the correct temperature for certain films. Other films require a different temperature.
Now, for your specific film there should be temperatures listed on the package. Have fun!
You should also note that most films will slightly change color when at the glue activation temperature. The color returns to normal as it cools. Not all colors show this effect, red and yellow seem to show it well, white and black not much. If you do not have good lighting you might not see the color change.
Phil