A pitman arm puller is a simple tool which can save you a lot of trouble - and may even prevent damage to your vehicle. Here's how it's used.
If the arm to be removed is on a very old vehicle, spray some penetrant around the spline and the fastener nut on the pitman arm. Allow this to work its magic overnight if possible. When you are ready, the fastener nut can be removed completely. Take your pitman arm puller and apply a little lubricant to the pressure screw before slipping it over the pitman arm and positioning the end of the pressure screw in the middle of the shaft.
Using a 17mm or 11/16 open ended spanner, apply tension to the pressure screw then take a hammer and strike the ball on the end of the pressure screw. It may take a few motions of tightening the pressure screw and striking with the hammer, but with some persistence the pitman arm will fall free of the steering box. Easy as that.