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.
For more tips on your outdoor tools check out The Family Handyman - Outdoor Tools
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Step 1: Tighten the Pivot Nut
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.
Step 2: File the Edge to Expose Clean Metal
Clamp the blade firmly in a vise. Examine the factory edge. Hold the file with both hands and mimic the direction of the bevel like a golfer taking a practice putt. Now move the file in one broad stroke away from you along the entire cutting angle. To reiterate, move the file in one direction, away from you. Don't use small, jerky strokes or you'll lose the factory edge. As you work, you can see the clean metal path left by the file. Adjust your angle as needed to file the entire edge evenly. Repeat this motion several times until you expose clean metal over the whole edge. Usually it'll take only about 10 strokes. Do the same with the other blade.
Step 3: Sand the Back Side of the Blade
Place a sheet of 300-grit wet/dry sandpaper on a smooth, flat piece of plywood. You'll be able to feel the burrs (be careful—they're sharp) on the back side of each blade caused by the filing action. To remove them, lightly sand the back side of the blade. Keep the blade flat and move it in a circular motion. After making several circles, pick up the blade and gently feel the edge. When the burrs left by the file disappear, assemble the blades and lightly oil the moving parts with 3-In-One oil. Then try a test cut with the shears. They should cut better than ever.
Step 4: Tip: Use Hedge Clippers
Use hedge clippers for cutting small diameter green wood. Thick branches and dry wood can bend the blades.