From your dashboard, click on Ticketed Events, then click on Manage. Next, click Attendees.

Find the name you wish to check in. Locate the three dots on the far right-hand column, and click it. You will see the option to check in that guest.

You will see the "Checked In?" column change from "No" to "Yes."

If you checked in someone accidentally, you can undo the check-in by clicking on the three dots and clicking "Undo check-in."