Get backside shifty's off small kickers on lock...
Then start on a wide flat box that you can ride straight onto, 50-50 it and about 1/4 of the way down the box turn the same way you did a for the backside shifty (front board position-I know its confusing) that way you'll get the feeling of sliding sideways going backwards and it'll let you turn the board a little more each time and work your way up to turning it a complete 90.
Once you get the board at turning at 90 each time you're ready jump on the box in that position. This is where it gets tricky because it's hard to get into that position coming straight onto the box from a flat base. You'll want to come from the side a little putting the box in front of you a little, hence the name-frontside boardslide. I like to make a little heelside set up turn to the outside of the box and come at slightly toeside from a small angle. Don't try and jump from your toes, stay more flat and pop up off both legs and do the shifty landing in the frontside position on the box. You'll be familiar with the feeling by now from learning them with the 50-50 on, and you'll stomp it no problem. Slide to the end of the box and ride away.