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Once you choose Mail Merge for Gmail. you should see the four headers appear on your Google Sheet
Email Address - This is where you put the Email Address of the recipients in this column
First Name - Place the first name only in this column
Last Name - Place the Last name only in this column
Merge Status - Leave this blank
You will be able to choose Draft
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One of the options you will have available is choosing your Draft. You will also have the option to View the Draft
At the top right of the sidebar, open the more options menu, and select Send a test email to myself: