In order to assign employees to shifts you first of all need to create shifts in the menu option 'Schedules'. See: 'How do I create a new shift?'
Now you have three different options to assign employees. One is to publish the schedule so that the employees can apply for shifts themselves:
Alternatively you (manager or admin) can assign the employees to shifts by dragging them from the employee list on the left hand side and dropping them into the desired shift.
The third option is to click on a shift and then on the + icon of the employees you want to assign to it. Here you can specifically look for employees that have stated their availability for the given timeframe by clicking on 'available employees'.