Full Use Case:
I would like to merge the first and last name of the Salesforce user that is currently generating the document into the template. How can I build this?

Consulting Reply:
You create a formula field on the object you are merging from. For this example,  lets use Opportunity object.  When you create the formula field call it “User Full Name” and when you build the formula you can insert a field to merge into that formula field.  When you go to advance and insert field you can select to merge from current User.  Starts with $User and would look something like $User.First Name.  After creating the field on the Opportunity you can view it on the page layout and see that it will just display who ever the current user is logged into salesforce at anytime.  And so then merge this new formula field you created into the template.