Add Merge Fields to a Word Document before adding a DataSource

Adding merge fields to a Word document without adding a datasource to it is not at all obvious when the mail merge button is greyed out. But to use MailMerge programmatically you probably want to do just that.

You can do it—unobviously—via Insert->QuickParts->Field:

Choose MergeField from the list on the left, and then you can type in your merge field name.

More recently, the “Field” button has been promoted (woo) to a top-level citizen of the Insert Ribbon, so now it' s a bit easier to find: