Your iron may differ but from the two that I tested it this was about maximum heat or slightly under.
With the front of your mask Facing up use the back corner of your iron holding it at about a 30-degree angle, with the bottom of the iron facing to the outer edge of the mask.
Then with a fair bit of downward pressure you can slowly advance forward around the edge of the mask, aiming about 5mm in from the edge.
Make sure you do a bunch of test runs until you find the right combination of temperature, speed and downward pressure.
It’s tempting hold the iron flat but it simply doesn’t work to bond through all the layers.