How do I do a mail merge with Gmail?
Labnol has a video tutorial and walkthrough.
The key is that:
There is an update to the Labnol article that shows how to add attachments - Labnol tutorial
(thanks to mvark)
Here's an excellent video tutorial on using gmail, google sheets and the free Yet Another Mail Merge add-in to do a mail merge:
You write a draft email in gmail, use this format to refer to column names in your google sheet when writing the email: