Start mail merge from Excel

I have customer data in an Excel spreadsheet. I have a number of different letters as Word templates that I send to my customers. From Excel I would like to choose a letter type and (on the press of a button) have the letter merged with the selected customer details and printed.

Does anybody know if this is possible? I have written the macro to open a new word document based on a template, but how do I merge specific contact information?

Thanks for any help