if you've ever watched a place kicker in football line up before a kick. you will notice he usually stands at the holder, takes like 1-2 steps sideways, and then like 3-4 steps backwards. if you notice, he is lining up how far each step is.
basically, take your stationary "push off" style throw where you push from your left foot onto your right foot.
to build a 1-step throw from that, keep your left foot where it is, move your right foot so that it is next to your left foot. your 1 step throw = move your right foot back to where it was, set it down, then push from your left foot onto your right foot.
to build a 2-step throw from that, start from your stationary throw, move your right foot back next to your left foot into the 1-step position. now, move your left foot back until it is oriented around your left shoulder. start your motion with your left foot cross step, when that foot hits the ground, basically execute your 1-step throw.
a full 3 s-step x-step is basically built in the same way. this way, every step you take contributes to powering your stationary throw position.