Inside, definately. If you try to do it on the outside, you're going to have gaps, and trying to fill those is going to be a challenge without ruining the whole look. Now, you COULD try putting the lenses on the inside, and then putting some clear plastic on the outside. That way, if you screw up, nothing bad happens. Will also save your sunglass lenses from any damage. ust a thought. Looks SICK!!! Love this, can;t wait to see it done!