There are two common ways SACLOS can work:
The sighting device can calculate the angular difference in direction from the missile position to the target location. It can then give electronic instructions to the missile that correct its flight path so it is flying along a straight line from the sighting device to the target.
These instructions are delivered either by a radio link or a wire. Radio links have the disadvantage of being jammable, whereas wire links have the disadvantage of being limited to the length of the wire.
With beam-riding SACLOS, the sighting device emits a directional signal that "illuninates" the target. A detector in the missile looks for the signal, either in the nose of the missile looking for a signal reflected from the target, or in the tail of the missile looking for the beam from the emitter. Electronics in the missile then keep it centered in the beam.
Radar was the most common form of SACLOS signals in early systems, as in the anti-aircraft role the target is typically being hit by a radar signal anyway. However a beam-riding missile has the disadvantage of flying directly at the target, which is often rather inefficient for a high-speed target like an aircraft. For this reason most anti-aircraft missiles follow their own route to the target, and do not "ride" the beam.
A more modern use of beam-riding uses a laser illuminator pointed by the operator. This illuminates the target, and the missile head has a detector for the frequency of light emitted by the laser and can therefore guide itself to the target.
Wire and radio guided SACLOS
Beam-riding SACLOS
Examples of SACLOS-guided missiles