A resource has to be selected to assign to an individual student. Clicking on Assign button next to the resource will navigate to Assign pop-up modal. 

To Assign to Individual Students

In the Assign pop-up modal, use the 'Assign to' dropdown menu to select the class the student(s) are rostered in. If the teacher has organized students into individual groups, the Groups tab can be chosen according to the teacher's preference.
 

Then select the Open to any students slider under the Assign to option to turn off access to any students

Next, select the checkbox(es) next to individual students' names within the Select Students drop down menu on the right.  

Finish entering the remaining assignment setting fields and press the blue Assign button on the bottom left to assign to your students.