So, I'd start by making a a new email and picking whatever standard template your company uses. Hopefully it is mostly blank, possibly with your logo.
For the Smart List in the Smart Campaign, you'll need a way to find all existing customers. Is there a field in Salesforce that shows that? It will depend on how your company runs things.
Then the flow step should be a basic Send Email.
This article might help:
http://community.marketo.com/MarketoArticle?id=kA050000000L7bf&src=univ