Open a draft from the invoice list and press Send. If you have no draft yet, start with Create your first invoice.
Ledgerly emails the client a link to the invoice and a PDF copy. The status changes to Sent, and to Viewed once the client opens the link.
Reminders
Turn on Remind before the due date in the send dialog. The client gets one reminder three days before the invoice is due, and nothing after it is paid.