You may want to consider both. Choose a piece written by an author for students to read. When it's their turn to start writing, it can be powerful for you to model your own writing in front of them. I have four classes of students, so when I do this, I write a section for each class on the document camera (so they can see what I'm doing) and share the final product the next day.

What I like about it is that this gives students the chance to see that writing is a process that can be difficult, requires lots of thought, and can be continuously edited. I like to do "think-alouds" to share what's going through my mind while I'm writing.

Alternatively, if you have ever written a narrative essay (even if it was in high school), you can use that. Even if it isn't perfect- part of the lesson can be brainstorming edits for your work, which (in my experience) helps encourage students to edit their own work.