they're still puppies ... they are well trained when they want to be... treats help!
and .... treats.... lots of treats...
... seriously we just start slow - let them sniff them, lick them, touch them. eat treats out of them. then slowly graduate to placing them on their faces. even then, not snapping them on right away and taking the lenses out, just getting them used to the feeling. then move on to snapping them, then finally (in short durations) putting them on with the lenses in.
patience and treats - that's really what most dog training boils down to....