Create a template using How to create a simple template guide. After creating template, go to Campaigns, then click on Create new.
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Scroll down and make sure you select Drag and drop editor. Then click Edit button as shown below.

There are two ways you can use templates in your campaigns. You can either use predefined templates from Template gallery or you can use your own from My templates. Be sure to create one and save it.
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Here, we have selected a template of our own from My templates by click on it.
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You will then taken to edit the campaign containing the template as shown below. After editing your campaign and making changes, be sure to click Done editing.
