In my experience, it worked best for me to call offices and ask for their office manager. This person should be able to help you find days that the doctor is free, and get you started with any paperwork they may require.
When you call I would just explain that you are a student at _____ (wherever you go), that you are interested in optometry, and wanted to know if you could schedule time to shadow Dr. So-and-so. It may also help to mention anyone who referred you to this doctor, or specifically why you are interested in them (like, their specific specialty that interests you, for example).
Good luck! I found shadowing to be very rewarding, and I hope you have a good experience too!