When you want to mail-merge data, first sort the data so the last submission is at the TOP of the sheet. Then select the header row and row two (or additional rows if there are multiple submissions you want to create docs for).
Once installed, you will find Save As Doc in your list of Add-ons picklist.
Naviatege to Save As Doc and mouse right to click Start. The Save As Doc popup window will appear on the right side of your screen:
Enter the file name you want it to have (you can change later) and select/deselect the settings. This is just what I have preset for my mail-merges but you can configure yours however you want.
When you're done, click the blue [Save as Doc] and a new Google Doc link will appear below the blue button. Open it and voilà!