If you are trying to replace tokens in (say) a memo field with the content
of the fields, you could use the Replace() function.
For example, you could replace the text "<<surname>>" in the field MyLetter
with the contents of the Surname field by setting the ControlSource of the
text box to:
=Replace([MyLetter], "<<surname>>", [Surname]"
If you have several tokens to replace, you probably want to call a function
that loops through the various tokens and returns the results to the text
box.