In order to send a reminder to a guest about the online check-in you need to create an email template which will contain a special key {ManageBookingLink}. This key will make that the guest will receive a unique link which will allow him to open his online guest account and perform the online check-in.

Please have a look here in order to see how to place correctly the mentioned key in the email reminder: How to send the information to guests about online check-in?