Tighten the Pivot Nut
Before sharpening make sure to take a look at the pivot nut to make sure its tight before you begin. If this nut is not tight it could cause the blades to separate slightly and tear twigs instead of cleanly cutting them. After tightening the nut check the tool again to make sure it still needs sharpened. If it cuts cleanly now then the nut was the issue. If it still cuts uncleanly check that the blades aren't bent. If they are bent unloosen the bolt and tighten the blades in a vice until they are straight.
Clamp the Blade
After the blade is firmly in a vice hold the file with both hands and mimic the direction of of the bevel and make sure to move the file in one direction, away from you. As you work you should be able to see the clean metal that the file will leave and do this until the entire edge is clean and sharp. Lightly sand the back of the blade while keeping it flat and moving it in a circular motion. When the burrs left by the fil are gone repeat the entire process on the other blade. After finishing you should try the blades to see how well they cut and remember that thick branches and dry wood can bend the blades so they should only be used for small diameter green wood.