Sometimes, we have a lot of ideas but don't know how to make it take form. Forms of poetry can sometimes help us with this issue. Epistles is just a fancy word for "letters", in which the piece is addressed to someone. Write a letter to someone or something. Think of your audience. In a poem, this can be a person (e.g. "An open letter to my mother"), a thing (e.g. "A letter to my favorite chair") or an idea (e.g. "To Sadness").
Examples of poetic epistles:
"Dear white America" by Danez Smith
CW: Anti-blackness, anti-black violence, misogynoir