Create a Template with Merge Fields
Step 1:
Either create a New document to use or open an existing Standard Letter, such as appears in the video above.
In the case of a New document, add all of the text, branding etc to your document highlighting locations in which to add Merge Fields.
Step 2:
Open the Place Merge Fields screen and use it to add Merge Fields to your document.
Step 3:
Use the Refresh All Document Data ribbon button to constantly check the data that is returning for your Merge FIelds.
Step 4:
When you are satisfied, Save the document anywhere you like including in a SharePoint Document Library. It can now be used as a template to do a Letter Merge