If the fork is a straight 1-1/8" diameter, the easiest way to do it is with a hacksaw. You can use a hacksaw blade that has teeth on both sides of the blade, which makes it easier to cut through the tube. If you have an older (pre-2007) mountain bike with a 1-1/8" straight steerer tube and you don't want to remove your fork from the bike, you can use an angle grinder with a cutoff wheel attachment. The trick here is that you must be careful not to let the cutoff wheel touch any part of the frame or fork.