To create a submittal template, you'll want to navigate to: Settigs>>Submittal Settings
From there, you may select drop down arrows at the end of each column to choose mail merge fields to include in the email. The "insert data" option can be used for the body of the email.
To specifically choose from available candidate information, you'll first want to select the "submitted candidates list" section as shown below.
Once included, you will be able to choose specific candidate related mail merge fields.
Additionally, you may select which documents you would like included in your submittal:
Select the plus sign to choose one or multiple document types (see article here on Creating Document Types)
You may then choose whether you'd like to allow multiple or make that document madatory.
Next, when you do submit a candidate, you'll be able to select which template you'd like to use from your picklist.