It's very simple and you only have to set it up once. Go into your ContactMonkey sidebar and click the Salesforce button at the top. Press "Click Here to Login" to Salesforce and enter in your login credentials.
Create a new email and address it to the lead or contact you wish to email. Then, over in the ContactMonkey portion of the menu ribbon, click Salesforce Templates. There you will be able to see all of your Salesforce Email Templates. Choose the template you wish to use and ContactMonkey automatically pulls down that template and populates all mail merge fields.
Please note this feature is only available for Gmail users.
Click here for a quick demo video