From the merchant portal, choose Settings from the navigation menu, and Notifications from the dropdown menu.

Choose the Notification to Customize

Find the section for Customer Emails or Admin Notifications (whichever one you would like to customize) and then click on the name of the notification, which will be shown in blue text. In the example, I am making changes to the Welcome Email.

Customize the Notification as Desired

After clicking the blue text, the Notification Editor will pop up. You can make changes to the template by typing directly into the editor. All changes should be made in BOTH the HTML and Text templates. You can view the two templates by using the HTML Preview and Text Preview tabs. The version that is on display on top is the version that you are editing at the bottom.

The default email includes template strings (For example, {{ }} which corresponds to your store name) that you can use when customizing your notifications. There are many template strings you can use, many of which are documented HERE.

Save Your Changes

Don't forget to press the Save button.