The "quick and dirty" way of syncing one's calendar is to not do it at all, and instead simply receive calendar invitations via email that, in most email clients, will be automatically added to the recipient's calendar.

This is the default method supported for new clients, and for many purposes, it works well enough.

A calendar invites are included in appointment notification emails. However, you can email clients' invites to their upcoming appointment(s) quite easily at any time:

One option is to send the invite from the Appointment listed on your Appointment screen:

However, if a client has several upcoming appointments with you, it's more efficient to send them invites for all upcoming appointments, which you can do right from their Client Manager.

Click the gear wheel for a given client to open up their Client Manager:

Expand Settings and go to Appointments from the left-hand menu. From there, you can click the button to email the client invite(s) to any upcoming appointments: