How to connect Salesforce to a single Business Unit
So you want to connect a Salesforce org to a single Marketing Cloud Business Unit (BU) and not the parent org? Have you found that Salesforce documentation does not tell you how? Did you spend two weeks googling "how to connect a Salesforce org to a multi-org BU?" No? Just me?
Just in case I am not the only one, here is your answer. When you get to the step where you connect Marketing Cloud to Salesforce (I recommend following the steps in this trailhead), switch your Business Unit!
There you go. That's it. That's the step they leave out of the trailhead and help documentation.
Here is the FAQ on multi-org if you are wondering why and when to use it.