How do I indicate that I've mailed my letters?

First check that your letters are marked " prepared." 

For campaigns with a fixed mail date: On the mail date, your buttons to mark your letters "sent" become active on your dashboard. They are grayed out until that date. 

For campaigns with a rolling mail date: You can mark your letters "sent" as soon as you mail them. 

Use the gray "Mark sent" button to mark a bundle of your letters as sent at once. To mark individual letters within bundles as sent, use the gray "View 20/20 prepared letters" button, and then use the right-facing gray arrow button next to each voter's name.