Hi, Sarah. An avatar is just a profile of your target reader. To create one, you do much the same as you would when you create a character for a story. You give it a name. You describe what he or she looks like. You give her likes, dislikes, and quirks. When you ask your avatar (or your target reader) a question, they’ll answer based on who they are.
Then you’ll know how to write for her.
Thanks for asking!