Unless you're a pro, it's best to stay away from using a grinding tool to sharpen your shears. The best and simplest was is to use a 10 in. long mill file. The file works very well on hedge and pruning shears. If you're not sure whether they're meant to be used for gardening, simply check the package.
Before sharpening, check the pivot nut. It could be loose, making the blades drift apart while cutting and tear the twig instead of cutting it cleanly. The nut should be snug with no play in the pivot. With the nut tightened, check the tool; if it cuts cleanly, it doesn't need sharpening. If it still cuts poorly, look down each blade to make sure it's not bent. If a blade is slightly bent, loosen the pivot nut and separate the blades. To straighten the blade, put it in a vise, slip on some thick leather gloves and tweak it until it's straight.