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 "Mark sent" button to mark all your letters in a bundle "sent" at once. This will move the bundle to the "Sent" column. 

To mark individual letters within a bundle as sent, use the blue down-facing arrow button next to "View 20/20 prepared letters." Use the right-facing gray arrow next to voter's names to move to the "Sent" column.