take your forward leg and dig your edge in on the same side of the rail. If your right foot is forward your right side of the box you dig your edge into and if your left foot is forward the left side and basically from there you just commit and spin
For frontside you're gonna wanna come on to the box more to the right if you slide right foot forwards or vice versa. scissor your skis and dig in that front edge. You should have a little forward momentum going now, so just hop up and spin 180 towards the front of the box.