I don't want anything. You are the one who came here looking for help. You mention that you have experience with Excel and Access. A function in Word is nothing like a function that you might enter into an Excel field.
Typically with a Word VBA "function," you will pass arguments to it and it returns a value:
Code:
Sub Demo()
MsgBox Add(2.5, 1.5), vbOKOnly, "SUM"
End Sub
Function Add(a, b) As Single
Add = a + b
End Function
Your function will return the value (and place it at the end of a document, a table cell, a content control or whatever target you define) each time you run it. Word has no built-in change event that automatically detects a user adding text (or specific characters in your case) or calls a procedure e.g., your function.
Now, if you put a content control adjacent to each of your objectives and use it to enter your "**" marks then you could use the Document _ContentControlOnExit event to call your function.
Private Sub Document_ContentControlOnExit(ByVal ContentControl As ContentControl, Cancel As Boolean)
'Call your function here.
End Sub