There are two ways that you can schedule an appointment.
The first way is by selecting Schedule from the top menu and clicking the Add an Appointment button. From here you should select the job you're scheduling an appointment for, how long it will take, the date, and the time that the appointment will start. You can also add notes for the appointment. Once you're finished, click the Schedule this Appointment button and your appointment will be created.
The other way to create an appointment is directly from the job your appointment is related to. From the Jobs list, find the job you'd like to schedule an appointment for and click it. On the job page, click Choose an action and select Schedule an Appointment. This will take you to the new appointment page where you can choose how long it will take, the date, and the time that your appointment will start, as well as add any notes related to the appointment. Once you're finished, click Schedule this Appointment and a new appointment will be created for your job.