xl prompt msg box macro

I have an xl sheet in which users enter data and submit to a spreadsheet. Tha macros are all recorded and I would like to add some vb. The users frequently forget to populate some fields (even though it is obvious what to do, doh!), so I would like a msg box to pop up if they forget to fill in any cells. My knowledge of vb is limited to recording macros and removing the obvious scrolls and stuff so please talk to me like I am a six-year-old. Thank you in advance.