The easiest way is to hold a tissue under your lashes. Fold the tissue and just lay it under your lashes then apply your mascara. There is actually a product for this, a shield. But I think it is 10 bucks! No thanks. If you need something more rigid than a tissue, use an index card. You could even cut it to fit the shape of your eye.
Hold the mascara wand vertically and sweep it back and forth on your lashes. That should work.