Its best if you try this trick on a ride on box for starters as its much easier.
For regular riders
1) Approach box with good speed
2) pop from your tail as you approach lip of kicker
3) Turn your sholders 90 degrees and let your board follow
4) Whilst in air load your weight onto your front foot.
5) Make contact with box flat based with your front foot inline with centre of the box
6) Keep front leg bent and straighten back leg (to make it look steezy lean towards front foot as far as possible)
7) Keep looking at end of box whilst sliding - when you reach the end pop off and land switch.