Step 1: Go to Edit/Advanced and click "Edit" on the Your Letter component
You have several options here. You can:
1) keep your letter editable or disable editing:
2) Write your own placeholder text that will disappear once a supporter starts writing their letter (this comes useful in actions when you're asking people to draft their own letter)
3) Keep your letter field resizable for your supporters or not - this largely depends on your website set up and length of your letter/petition.