To enter an employee availability pattern, go to the Availability section in the blue bar at the top of the page.
Once there, you can use the drop-down menu to switch to a different employee. You can then use the blue Plus button to add a new availability pattern.
Below is a video on how to add availability patterns. It's from the point of view of an employee, but the information on the tool itself applies if you're adding availability on behalf of an employee.
You can also edit Employees' Availability on Mobile, see below for more information: