There are two ways to use the "Fast Adjust" feature on the Gen 2 Solid Mask:

 

Option 1:

Simply place on your face, tighten the two adjustable straps, and adjust to fit.
 

Option 2:

If you do not want the adjustable strap excess left after your adjustment, tie a knot at the connector loop, check sizing again, and then cut any excess strap

with scissors.