I started hitting these ealier this season, and the best advice i can give you is faster is better (generally safe to say about boxes and rails). try to jump with a small angle towards the box (not a side hop), and definately start by jumping onto it with your toes facing the box so you can easily see where you need to land. The first time i did it, I rode it for a sec, then came off early on the other side. This is most likely what will happen to you, but as you hit it more and more you will get better at landing with very little sideways momentum so you can stay on for the whole duration of the box. Going faster also means you won't have to ollie as much cause you will land further down the box, and the faster you are going, the better chance you have of eliminating most sideways momentum that you have cause you are approaching the box from the side. typically the way they are set up, you don't need to pop huge ollies to get onto them, but it can't hurt to ollie higher than lower the first few times you hit the box, until you figure out just how much you need to pop so your board clears the box.
I would highely recoment hiking the box your first day and you will have it down pretty quickly, it helps to hit the box quickly in succession.