an ILS back beam approach uses the ils beam for the opposite runway. You cannot use the glideslope feature as the glidesope is calibrated for the opposite runway. You can use the localiser signal, but if using flight director guidance it will turn you the wrong way to establish on the centreline. To fly one, the flight director is switched off, and rather than steer towards the displaced centreline beam bar to correct to the inbound course, you steer away from it to drag the beam bar back into the centre. Simplified and basic. Modern systems can compensate and show correct displacement signals at the flick of a switch.