i did that once. i used a model. i wish at teh time i had kept it to a single application, ie, just eyes, or just foundation, or just lips.
you get to be more informative, which is the most key part to any speech, and no great makeup application, when explained, takes less than 15 minutes. you need time to explain the products what they are used for and how to apply.
also, when using a model, its MOST important to say what you are doing, as in if you are doing eyes, state that you are doing an eye application that opens her close set eyes or how to give lips a plumper look when you have thin lips or how to get even looking skin when you have redness to your face.
always do a rehearsal with your model to make sure you are doing exactly what you are stating. and to make sure your colors look right on her.
edit:
oh- i also did a handout of face shapes, and the color chart and how complimentary colors work, and undertones and the color foundation you should be using and how 80% of all women can get away with a medium beige foundation...
if you do eyes, you can show the brow, the waterline, the inner v, the outer v, the main lid, the crease line. you can pass out super cheapo crayons and tell everyone to choose 3 colors and practice a look on their eyeball.
as you can tell, i am pushing eyes because those make SUCH an impact, even when doing an everyday look, and there is a lot or as little as you want to go into.