This article describes how to insert merge fields into email subject lines.
Insert your *|merge field|* in the Email Subject box.
In the example below, we have a column called Name in Google Sheets, so we can use Att: *|Name|* as the email subject.
That's all! Your emails now have their subjects personalized.
In the example below, our email subject (highlighted in red) is Att: Samuel Reeves.