I have only made eye glasses once a few months ago. But how I did it was to use gum paste and create two eye glass fronts exactly the same, and then a left and a right ear pieces. Let them dry completely flat. For the lenses I made them out of gelatin prepared like you would do for making butterfly wings.
Once everything was dry I adhered them together with royal icing colored the same as the gum paste. However, use as little as possible around the gelatin or it may warp.
Below is a picture of the cake I made the glasses for. HTH