Begin by strongly driving up while keeping your back flat. The first part of the pull is slower. Then as the bar passes the knees the knees and hips will drop under the bar. Continue to drive up strongly so that the ankles, knees and hips are all at full extension while the shoulders complete the upward momentum by shrugging.
As the bar continues to travel upward, drop your hips and pull yourself into a front squat position. Once the bar is resting on the shoulders drive upwards just like a front squat.