Edges: File the edges to keep them sharp, as this will make the board cut through the snow faster and turn better. To do this, find the board's 'effective edges'. These are the two parts that touch the ground when you lay a board on the floor. Always make sure that you stop filing 10cm before the effective edge, but taper it off, don't just stop dead.
Top Edge: place the board on its edge in the vice/bench. Using the 89° angle iron as a guide for your file (see pic), make long, smooth strokes from nose to tail to remove any dents. Use a diamond stone to get rid of any dents etc that the file can't handle.
Base Edge: once the top edge is done fix the board on its back and, using the file with the masking tape wrapped round it, file the base edge. Edges need to be filed at an angle of 89°, so by wrapping a little tape around the file, the correct filing angle is achieved.