Your customer's Google Glass comes with a mono earbud. They can also purchase mono earbuds and stereo earbuds from the Google Store.
Here's how your customer can put it on:
Plug the earbud in to the the micro USB port (the same port used to charge Google Glass). If your customer is putting on stereo earbuds, advise them to wrap the cable of the left earbud around the back of their neck and the cable into the earbud to create a loop where they can adjust it so it presses into the curve of the ear for a secure fit.
Slide the cable to adjust loop size for a better fit.
If your customer feels like switching it up a bit, they can change the colour of their earbud by replacing the cap.
Earbuds bought from the Google Store come with interchangeable coloured caps. Your customer can replace caps by twisting them onto the outside of the earbud. All they have to do is turn left to loosen, and turn right to tighten.