JenniferEdieRose, I've got a round face too, and there's a couple of tips I picked up to slim the face - I'm a makeup addict, so these are tried-and-true
To make your nose thinner on the side, put some illuminating cream or highlighter on the sides. The reflection you get from the shimmer draws attention away from the width of the nose - from the front, especially, your nose looks much thinner because the focus (and the darker shade) is more on the front of your nose. It really works.
For the ball in front of your nose, dont do this- shimmer will just draw attention to it. Keep it uniform, the same color as your face.
For contouring, having a contour brush helps, but I dont bother. I put blush on the apples of my cheeks, and then I take any large brush, dip it in a shimmer tanning powder (I use one from MAC), suck in my cheeks a little, and brush it just below my cheekbones, from the middle of my cheek (under the blush) to my hairline. I make it SLIGHTLY darker in the hollows of my cheek, than the area by my hairline, because if it was all the same color, it'd look scary. Plus, by making the cheek-cheekbone area darker, it makes your cheeks look much thinner.
Hope this helps, and if I wasnt concise enough, which I often am not, just let me know, and I'll clear it up!