Watch Wayne Goss's full tutorial demonstrating this technique here, and of course, be careful, because lash combs are made of metal! If you don't have a lash comb, you can find them for around $5 at Target.
P.S. If you try this, make sure to clean off your lash comb after each use (you can just wipe off with a makeup or baby wipe, or use a bit of makeup remover). If the mascara dries on, it will be impossible to swipe through your lashes so easily! And it may start growing bacteria in a spot that's really close to your eye on a regular basis.