If you really are artistically challenged, then cribbing will help to start with. I always suggest that FIs find very simple images from text books or the interweb and make nice neat copies onto A4 sheets (trace if you have to). These can then be laminated and inserted into your instructor notes. If you are repeatedly drawing the same pictures, you will get better at it.
Also a well equipped briefing room should have a few aids that will help. These need be no more than tops off coffee jars, broken rules out of lost property etc.. Neatness with accurate circles and straight lines will work wonders with presentation.
Sometimes too much writing can make a good diagram look confusing. Consider using a straight edge to put arrows on the picture, and then put all the text away from the image on the other end of the arrow.
Hope this helps.
XO