I haven't done this on a fork but i've used this technique on a rear triangle. Put the fork in a vise, take a two by four and put it between the fork blades legnthwise all the way down to the fork crown. Pull towards you to bend one blade. Be careful when bending, the idea is to slowly increase the force that you are applying until you feel the metal bend. When you feel the slightest give of the metal, stop, check how much it bent and bend more if necessary.