How do I mark my letters "sent"?

First, check that your letters are marked " prepared." 

To verify the mail date for your campaign, please check your dashboard or the downloaded bundle cover page. When the mail window begins for that bundle, the buttons to mark your letters "sent" will become active on your dashboard. They are grayed out until that date. 

Use the blue "Mark sent" button to mark a bundle of your letters as sent at once. To mark individual letters within bundles as sent, click the down arrow next to the gray "View 20/20 prepared letters" or "View 5/5 prepared letters" button, and then use the right-facing gray arrow button next to each voter's name.