In order to assign employees to shifts, click on one employee in the list on the left hand side in the schedule and drag him into the relevant shift (drag&drop). Another option would be to click on the existing shift and to add employees by clicking on the green '+' icon next to their names.
If you are not sure if your employees are available, you can publish the schedule. This way they can apply for shifts and you only have to accept and assign them. This saves a lot of time and nerves ;)
Employee drag & drop:
Assigning employees to shifts by clicking on the '+' icon: