To answer your question about where to put your code to check those cells:

In the Select Case block:
Case vbYes
'means the user wants to save the file.
'doing your check here before saving the file will cancel closing the file
With Worksheets("Drawing Notice (Main Sheet) 1")
If .Range("C16").Value = "0" Or .Range("C16").Value = "" _
Or .Range("E16").Value = "1" Or .Range("E16").Value = "" _
Or .Range("G16").Value = "1" Or .Range("G16").Value = "" _
Or .Range("I16").Value = "1" Or .Range("I16").Value = "" _
Or .Range("C17").Value = "" _
Or .Range("F17").Value = "" Then

'display your message
MsgBox "You must complete the highlighted fields."

'set the value being passed to Workbook_BeforeClose() and exit
bBeforeShutDown = True
Exit Function
End If
End With

'If all the cells pass the test then we save.
Wbk.Save
'Placing your test "as is" in the BeforeSave event will handle when the
user saves.
'If the conditions pass your test in either place, normal behavior occurs.
'If the conditions fail the test in either place, the user is returned to
Excel to resume.

As I said previously, I don't normally handle Workbook events from the
ThisWorkbook module. I did earlier on but I switched to using Auto_Open and
Auto_Close procedures and thus, revised all my code snippets accordingly.
That's why the earlier example didn't work for you "as is", ..it was intended
as a suggestion of how you could handle the task. After reading your response
to my reply, I realized you needed more explicit help with it. I hope that
has been accomplished!

Regards,
GS