Find or take a picture of eyeglasses in about the same orientation as the person.
Use a picture with a good solid background so that selecting the glasses will be easy.
Use the color picker selection tool to select the glasses, then move them onto the image of the person.
Use the Free Transform tool to resize and move them into position. I also used the Transform Warp tool to bend the earpiece into position.
Add a layer mask to hide the parts of the glasses that wouldn't be visible (behind head, far side of nose, etc.)
Add a drop shadow and make it onto its own layer. Distort and mask the shadow to follow the face.
Optional: Add another layer style: bevel and emboss to help minimize the "cut out" look.
Add a layer for eye glass lens reflections.