Wax your Back
It’s likely that your edge sharpener has several angle options. 90 degrees is the standard setting for beginner / intermediate riding. 89, 88 and 87 degrees are progressively more advanced. The more acute angles will give you more bite, which is handy on hard-pack and is often used for racing.
Once you’ve decided on an edge angle, take a permanent marker pen and draw down your side edge. This technique is handy while you’re getting used to sharpening edges, but not always necessary. When the line’s gone, you’ve sharpened the whole edge.
You should only sharpen the concave section of the edge. To see where to sharpen more precisely, place your board right way up on a flat surface, the area you want to sharpen is between where the board touches the ground at the front and back (assuming you have a regular camber board).
The file in your edge sharpener is probably one-directional. There should be an arrow on the side of the file that indicates the direction it should go. Arrange the file so that it is pointing in the right direction for you to stroke from nose to tail on your first edge – ideally most actions you take when waxing and edging go in one direction, from the nose to the tail of the board. Now apply pressure with the file on the side edge and drag it along the length of the edge. Do this several times until your black marker pen line has gone.