OK,

I have a letter with a blank bit for the recipients Address in the top left. I have a list of addresses. What button do I press to magically transfer the list of addresses into the letters for each of the recipients?

Thanks

<Tools < Letters & Mailings < MailMerge Wizard

Your list needs to be in a data format so that Word knows which bit to put in each line each time.
Either have it in a table format or in Excel.
It will recognise Outlook address books as well.

The 'Help' information in Word is very good. Type 'mailmerge' into the help search box.