The information about the online check-in should be sent to the guest by e-mail. You can create a separate template for it in case you want it to be independend of the booking confirmation and you can do it through the booking confirmation template.

Here you can see how to create a teamplate: Setting up automatic emails

In both cases you need use {ManageBookingLink} key:

Mark the text you want to be connected to the online guest account, e.g CLICK HERE TO MANAGE YOUR BOOKING. Righ click on it and choose Insert/Edit link:

You will see a small Insert Link window. Put the key {ManageBookingLink} in the URL section:

When the guest cliks on the link...

He will be taken to a unique address where after putting the last three characters of his booking ID he will be alble to perform the online check-in.

The email can be sent automatically depending on the conditions: Scheduling automatic emails

Or can be sent manually: