With one-arm, grab a kettlebell and hold in a front rack position. The movement will be on the same side (Ipsilateral) as the side which is holding the kettlebell. Now even though you're lunging backwards with the opposite leg, the movement should me mostly controlled through the leg that's left in place. Here you see me doing the movement through my right leg, while holding it with my right arm. 

Next, you always hear to keep your spine straight. This doesn't mean drop straight downward and keep your spine perpendicular to the ground. If you notice my spine has a slight slant foward, which allows me to keep it straight and aligned from my knee, hip, shoulders and neck. This is to truly mimic and develop the similar movement of a squat.