create a placeholder attribute of the document type to hold the merged document eg Customers.letter (note in the example I've used Members instead of customers because I've just pasted some of my code for speed
create your word document with the necessary tags eg <<Customer.Name>> << Customer.Address>> etc etc, you can also use the LIST_LINE functions to pull in sub tables,
create a process that a) Finds your document template eg FIND UserDefDoc WHERE UserDefDoc.UDD_Name='Renewal Letter'
b) finds all the customers you need to merge into the document eg FIND Member_REnewal_Email_List
c) chains to process 2
create another process(process 2) using Customers and userdefdoc as input. It just needs one line
Members.RenewalLetter=UserDefDoc.UDD_Name
The merged document is stored in (my case) Members.RenewalLetter
You can then print it, attach it to an email - do whatever you want with it
Hope this helps