One of the options you will see available within your workflow templates is titled "To Do".  
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You may add this to any phase of your template by pressing "Add > To Do" :
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After adding this to your workflow, you will have the option to give it a name, due date, add a note, and set what should happen next after it has been marked as completed:
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When setting the "Due Date", you will have the option to select how many days before (or after) the To Do is due in relationship to the workflow being activated, the project date, or dependent on the previous item in the workflow.
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The "When Done" field is used to trigger a specific action such as sending an email, questionnaire, invoice, etc.. once the To Do has been completed. 

Depending on your selection here, you will be able to choose from the related templates and prompts (such as giving you the option to approve prior to sending or to send automatically).

​Tip: 

Use a To-Do as a "review" before sending an email automatically. 

If you set the To-Do to "Send an Email" When Done, (Automatically) It will send as soon as you check it off. Allowing you to review the project first, then send with 1 button. 

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KeywordSearch: Workflow Item, Automating, Workflow Page, To-Do, Workflow