If you switch a calibration lamp on (or in twilight if you don’t have the calibration module) you can see the slit in the guider image. I use PHD2 for guiding and note the XY position in pixels of the point on the slit where I want to place the star. If everything is screwed down tight the spectrograph is sufficiently stable that I can then set that location in PHD (or an offset from that location if guiding on a field star, the offset having been measured on a stack of longer exposure guider images)
Robin