If they are very dull, nicked or blunted, I would recommend a professional sharpening to start with. Once they have a good edge, it is a simple matter to touch them up with a sharpening steel when you need to. If you are doing it yourself, a combination whetstone is good. To start with, coarse side first, keep an angle of 15 to 20 degrees for a razor edge, 20 to 25 degrees for a chopping edge, then finish with fine side. Move the blade across the stone as if you were shaving a thin piece off, starting at the heel and finishing at the point, always moving the knife away from you. Make sure that you treat both sides of the blade the same.