To do a proper popped bigspin, having popped 3 shuvs helps. Whip your shoulders around and pop straight down without too much scoop, maybe a little scoop but definitely make sure your tail is hitting the ground clean. Imagine you're trying to spin the board around really fast instead of a slow and high pop shuv... I guess 'zippy' would be a good way to describe how the pop and spin should feel. Feet close together, make sure your trucks are leaning on the heel side or at least flat, it makes the pop work a lot better and makes you not shift too much laterally.
If you have the problem where the board is doing the popped 3shuv just fine but your body cant do the 180 and only does a 90 or something, try look at the nose of your board the whole time. It kinda tricks your head and shoulders into turning the full 180.