er... where to start... with flex8's, you need to threat the strap through the ear peices, before you attatch the strap to the frame... once both ears are strapped up, then proceed to insert a corner of the strap into the frame, i sometimes use a key, if i have a particually awkward strap. once a corner is in, you will find it easir to thread the rest of the strap in. always pull the strap[ in further than needed, to straighten it up, then start to pull it back out again, taking care not to snag the foam from the frame.
repeat these actions on the other side of the mask. good luck.