The way I do it is put the head on a 2 burner hot plate and heat the head pretty hot. Usually just as it begins to smoke a little I'll remove the head from the hotplate and immediatley hammer the guides in with an air hammer with a valve guide tool. This might sound crude but it works. Very seldom do I have to ream the guides only sometimes just the hammer edge has to be cleaned. Be careful not to knock them in to far I always mark them before installing them as not to install them too far.