Write a thank you letter to your internship supervisor and submit a copy of the letter. 

Thank you Letter to Internship Supervisor

Directions
Write a thank you letter to your internship supervisor and submit a copy of the letter. This letter must be an actual typed letter and not an email. An email is not nearly as professional as an actual letter. Be sure to note the most valuable information you have gained from your internship experience and include thanks for other staff members or departments that you have worked with.

It is imperative to check for spelling, grammar and so forth. Remember that one day your internship supervisor could be interviewing you for a position.
Complete your own internet search to find sample letters that you can use to guide you in composing your own letter but be sure to use your OWN words. Do not copy directly from the internet, as this is not only unprofessional, it is inappropriate.