I wanted to draw a series of mannequins in my next drawing, but I don't know ho to go about it. Should the hair look realistic or like it was molded out of plastic? Should their faces look semi-flat? (like the eyelashes, eyebrows) Any help would be appriciated!

I would just get a bunch of photos of mannequins, and draw them just as you see them. Google image seems to have a lot, but you might get the best by heading over to Macy's with a camera of your own.