First things first, you *have* to know how to ollie onto boxes. If you haven't already, then maybe go back to those 50-50s and learn some presses (because in the process learning to press you will learn to ollie onto features).
Ride on boxes can teach you some terrible habits, they make it so you don't have to worry about a lip. This in turn creates a habit of slide-spinning on (you know, like you start to rotate into the slide while your board is still on the snow). Ride on boxes also make it difficult for a new rider to really gauge the transition from snow to box.
So approach your boardslide like this:
First, get ollieing onto a box proper.
Then, you want to pop into 50-50s and start squaring your shoulders (parallel to perpendicular; "I" to "T") and using your arms to shift into the slide towards the end of the box. This will get you use to both how sliding sideways feels and how your weight needs to be to avoid catching your edge. The more comfortable you become then the earlier you can shift into the slide... now you are getting close.
Nailed that? Now finish her up. Pop up like you were going for a 50-50 and shift your upper body like you have been. Once you know your board is above the feature, follow through with your lower body into the slide. This last part obviously happens quick. I wanted to break it apart to emphasize the shoulders to legs rotation.
Don't forget to **bend your knees**, it really helps to keep you flat based.
The method kylekilljoy mentioned will do you a boardslide, but breaking it down and really experiencing how the shoulders and arms drive the rotation will set you up for switch-ups, spinning out etc.