Go to Setup > General then click on Documents. Press the Add Document button to create a new contact.

In this page you will design documents that you would like to be sent to the customer (example - Confirmation letter, cancellation letter, detailed receipt, etc). Type in the name you would like for the document in the Name field then in the body of the document you can put in the message and details. On the right hand side of the page is a table for Reservation Variables. This is a legend you can use that will populate values from a specific reservation simply by copying the variable within that table and pasting it into the document body. "{CHECKIN} and {CHECKOUT}" for example will display the check-in and checkout dates for a reservation. Click the Save Document button once you are ready to save your document.

You will be returned to the Documents main page and now see your newly created document. You can always go back and edit an existing document by clicking the Edit button within the table that corresponds with that document.