Be sure to obtain in writing from client that they wish to cancel their booking. Be sure to inform them of any penalties, administrative fees and insurance coverage associated with the cancellation. Also, be sure to point out where you originally stated this information to them (i.e. in proposal email, in confirmation email, etc.
Also Obtain written acknowledgement from client after the cancellation is complete.
Call vendor(s) or follow vendor's procedures for cancelling the booking and request instructions, if any, for further action.
Update your records regarding payments and the like in your CRM. This is for your records only.
Send written confirmation and related documents to client.