Create a new template by opening your DealMachine mobile app or logging in with your computer to https://dealmachine.com
Click the menu button in the top left (mobile only)
Click Mailers.
Click Templates.
Click create new template. Click Save.
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If you want this template to be the default for new deals, check out this article:
How do I set my default/campaign mail template?
If you want to apply this template to existing deals, check out this article:
How do I change the mail campaign or mail template for a lead?