I have had many people ask how to attach straps to their blank masks and this is how I make the adjustable straps for my ready to wear masks. This way it can be worn by different sized faces.This uses a 1" metal bra slide and 1" flat elastic about 10" -12" long with felt squares and high heat hot glue. I use the high heat since it's a bit stronger than normal craft hot glue and recommend the Surebonder high temp industrial gun with Steinel high heat glue sticks.
Keep in mind you may still need to use heat ( please refer to the section under "shaping" here ) to soften and then widen a mask to get it to fit you comfortably. Upholstery foam or soft cloth hot glued inside the mask will also help it fit better.
You can also just attach one elastic strap without a slide that goes across your head to the inside of a mask if you will be the only one wearing it and won't need to adjust it. Just measure the strap against the back of your head ( remember it's going to stretch) and hot glue to the inside of the mask minus the loops like in the tutorial.
Hope this helps a few people!