^^^ its a little easier to understand if you describe it using toes. So
FS spin out, I set up for the spin with my back foot toes up, and my front foot toes down, then when I want to initiate the spin, I pop and turn.
BS spin out, Set it up opposite footing, then the same thing, pop and turn. For bs spins, I find it more helpful to focus less on the scissor and just focus on being solid on the rail, then pop and look around to the back. key being popping and turning in a solid manner.