We all play sex educators in our normal lives, whether it is explaining a menstrual cycle or explaining the suggestive lyrics of a song on the radio.
Addressing sexual orientation, homophobia, masturbation or marriage are all acceptable examples of topic you can write about. Make it personal.
No APA citations necessary.
Pick an age group and a sexuality topic to address. You can write it using suggestions from the SIECUS guidelines if you feel like you need guidance or appropriate messages (available in web links).
The letter can be intended for a real person or a fictional one (even your younger self). Be medically accurate, age-appropriate and fact-based in the information you provide. The letter must be at least two pages long. Please start it like a letter (Dear Suzy, Or To Whom It May Concern).