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 "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