This quick step-by-step tutorial will guide you in your first steps.
Let's take a look on the sample data. Keep in mind that the email addresses and card IDs are generated so you have different data in those two columns as on the picture below.
The email column contains recipients. Then there are three empty columns which will be populated by the add-on. Data from the last three columns will be merged into Gmail messages.
Add-on created a sample draft named Membership Card. When you got to Gmail Drafts you can open it and see what is inside. You will notice a couple of merge fields there. Merge fields will be replaced with values from columns based on the mapping configured in the mapping editor.
Select process all rows and hit the GENERATE button.
Once the merge job is completed column Draft Created contains timestamps and Draft ID draft ids.
Go to Gmail Drafts folder and you will see there eight new drafts named Membership Card. Take a look.
Switch to Draft to Send mode. In this mode add-on takes draft ids from the Draft ID column and sends the corresponding drafts. Once a draft is sent add-on removes it's draft id from sheet. Start the merge job and click on SEND button.
Add-on will send email to Mail Drop service. Once the job is done check your Gmail Drafts folder. Only the template draft left there. All drafts were sent and now you can find the messages in your Gmail Sent folder and data in sheet should look like this. Email Sent timestamps set and Draft IDs removed.
Now you can check the messages in Mail Drop service. Just keep in mind that it might take few moments till a message appears there. Take the first part of email address before @ and paste it in the input box in the top right corner and click view inbox.
The first email address starts with 6nsm (these email addresses are generated so you will have different email addresses in your sheet). Paste that there. You can use the Mail Drop service to test your emails also in your future merge jobs.
And that's it. You just ran your first merge job. You can edit the Membership Card template and you can try to add new merge fields and you can try to map them in the mapping editor. All should be self-explanatory. If not and you are stuck and you need help you can ask a question in this Google Group.