Start w/ butter or disco box.  Those are the ultra fat ones.  Best is one that you can just ski onto without having to gap to it. Practice 50/50.  That means going straight on the box; no 90 on.  Think wide stance, but no wedge.  Practice settling or getting a little lower when you are on the box.  Once stance starts to feel solid, start 50/50, pivot feet to 90 and then back.  If that feels good, jump up and pivot your feet like you did from 50/50.  Try to land in the same solid wide stance that you had when you pivoted fro 50/50.  Main keys are to keep wait evenly dispersed on both feet, settle a little to lower your center of mass.  Most important, keep your dick between your feet, assuming you are a guy.  If your hips cross over your uphill foot, you will go down.  Sliding a box or rail is not the same as doing a hockey stop.  Good luck and have fun.