Errin,
Using the Assembly Line Method, here's how I do it:
- 1st apply your chinstraps and Mod Podge them down and let them dry in place.
- Use your Exacto blade to cut the PAINTED facemasks off the sprue.
- Hold mask between thumb and forefinger and place on the end of your paintbrush. Apply pressure down and around the brush end. Mask should easily curve. I generally face the bottom of the mask toward the bottom (narrower) end of the brush. Use a letter on your brush as a guide to do all of your masks the same way. I do masks one group at a time: 1. QBs, HBs & DBs; 2. FBs, TEs, LBs and DEs; 3. OL and DL.
- Brush on some Mod Podge (a lot or a little) along and around the edge of the helmet - where it meets the face.
- I use my fingers to put the mask in place. Sometimes I gotta wiggle it, sometimes I don't. (TWSS!!!
) Try to get the mask OVER the chinstrap - that's why I let the chinstraps dry 1st. 
Or else you gotta use your Exacto to push the chinstrap UNDER if it slides. - Once you get the mask and chinstrap situated, tack it in place with some more MP - from temple to temple.
- Bada Bing! You're done! After all decals are in place I then slop on Model Master Gloss head to toe.