How do I properly sharpen a dull kitchen knife?

Start by choosing the right tool: a whetstone gives the best results, though a pull-through sharpener or honing rod works for quick fixes. With a whetstone, wet it with water or oil, then hold the knife at a consistent 15 to 20 degree angle against the stone. Push the blade forward across the stone as if slicing a thin layer off it, working from heel to tip. Repeat on both sides evenly, starting with a coarser grit and finishing with a finer grit to polish the edge. After sharpening, use a honing rod to straighten the edge by running the blade down it at the same angle a few times. Test sharpness by slicing paper or a tomato skin cleanly.