I use Astroart 5 which has a slit guiding facility. You place a cross over the image of the slit in the guide camera window and the software will then keep the star centred on that point. It seems to understand that the image of a star in the slit has two lobes above and below the slit and manages to keep it reasonably stable within the slit, subject of course to the vagaries of seeing. I have used it for years and am very happy with it.
David