Starting from the working template in the Letterhead example:
Information not only can come for the work product (case, property, etc.) but also from system level information. These fields always start with the word System as the first level of the field merge codes.
Here we are gathering your office name and address information, as well as today's date. This generates the following:
Note that there are formatting codes available for merged fields, a common one is to use [\System\Today@formaldate] in place of [\System\Today] which would generate April 2, 2010 in place of the 04/02/2010 shown.
All possible system fields are shown in the field merge codes under the Global Fields section.
Next: Address Block