Back foot should be resting on the tail with the big toe in line with the concave, pointing towards the nose at an angle. Then when you throw your shoulders into the rotation and pop, you apply the pressure down right there and the board should stick to your back foot with the rebound off the tail as you come around. It's a bit similar to how you pop and scoop 360 flips except you go with it and don't flick. Proper shoulder movement is different from doing a 180 then pivoting, your upper body has to lead the rotation and you have to think about the second half of the 360 in advance to know where to look and land. Doing backside 360 slides on the ground or step hops on flat (that come around in just one motion) will teach you the flow of that trick.