Your job as a spotter is to make the lift safe. This might mean simply being there so the lifter feels safe, or it might mean something more tactile, like helping rack and unrack the weight. The video below is a complete failure on so many levels.
There are two ways to spot a back squat. You can spot by the sides of the bar (one person on each end and outside of the rack) or from behind the athlete.I am a huge proponent of spotting from behind the lifter. In addition, squat racks with sidebars are a great benefit with and without a spotter as they catch any failed rep or falling bar and thus save the spotter and the lifter from injury.
Let the lifter back away from the rack and settle into the starting stance or set up. Stand behind your lifter and follow him or her down (in a similar motion as the squat itself). Have your hands up and under the armpits and on or near the chest (not touching unless necessary). Your role here is to help the lifter maintain a raised chest as the tendency when fatigue sets in is to collapse the core or trunk forward over the quads. By your guiding that chest up, the lifter has the advantage of maintaining form while still fighting for those last important strength-building reps. Also, help the lifter re-rack the weight by guiding the bar back with him or her into the proper rack position and securing it into place.
via Break Muscle