How do I mark my letters "sent"?

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. 

To mark your letters "sent": Use the gray "In the mail? Mark letters sent" button to mark all your letters "sent" at once, or the right-facing green arrow button next to each voter's name to mark your letters "sent" one at a time.

"In the mail? Mark letters sent >" Button

Video Demo