Cruising around the mountain on your fresh-out-the-pack board with shiny new edges is all fine and dandy but eventually after inevitable wear and tear you'll find your edges aren't the same as when they first started out. Gently rub your thumb along each edge. If they feel dull, rounded or burred, it’s time to sharpen them.
For this you will need a proper ski or snowboard edging tool. This has a built-in guide enabling you to maintain the correct angle.
Start with the base edge. Applying light pressure, move it smoothly down the board from the nose to the tail.
Do this a few times, always in the same direction, ensuring the file is flat on the base while the guide is flush with the side edge.
Your strokes should begin and end at the wide points, just before the board curves up towards the tip and tail. The ends of your board don’t need to be sharpened – in fact they’re usually de-tuned to prevent twitchiness.
Once you’ve done both base edges, switch the tool around so that it’s in the side edge position. Depending on the tool, you might be able to set the exact angle that it intersects on the corner. A standard edge tune is 90 degrees, though for more aggressive carving or hard snow you might opt for 89 degrees or less. (If you're really serious about your edges you can even adjust the angle of the base edge too, though this level of precision is usually the realm of racers).
"Begin at the wide point near the nose and work your way to the corresponding wide point at the tail"
Again, begin at the wide point near the nose and work your way to the corresponding wide point at the tail, always using the guide to keep the file flush.
Pay close attention to any sticking points. This usually indicates a burr in the edge, so go over these areas a few times until you can run the file smoothly all the way down using only light pressure.
Now check the edge by gently rubbing a thumbnail across it at various points. If it’s sharp, the corner will scrape small bits of nail off without much effort. Job done.