How to make a sequence diagram for this?

A group diary and time management system is intended to support the timetabling of meetings and appointments across a group of coworkers. When an appointment is to be made that involves a number of people, the system finds a common slot in each of their diaries and arranges the appointment for that time. If no common slots are available, it interacts with the user to rearrange his or her personal diary to make room for the appointment.

• First step is to think about what objects you have in the system. Make a list of candidates.
• Now think about how does this scenario get initiated. Where does the message come from? Draw that incoming message.
• The next step is to think about which object is going to receive that message. Now in order to do the work that object will probably need to talk to other objects. Sketch out the other object with an arrow and a message name.
• Keep thinking about the sequence of messages and the objects to which the messages go - and try sketching them out one at a time.
• Don't expect to get it right first time. Try sketching out several approaches.