In order to do a powerslide to bigspin, you really need to know how to do a powerslide for starters, and also a bigspin. It's very important that you are comfortable doing a bigspin from fakie because after do the powerslide, you will have done a 180 and you need to toss the bigspin backwards. It's not a very easy trick, but, if you're comfortable with a bigspin or getting into a powerslide, then you shouldn't have too much of a problem landing this trick. It's just like a heel-side check, except you're going to be kicking out a little further, so your board does a full 180 degree rotation.
After that, you're really going to want to kick out from your back foot and toss that board with your front foot. So, your back foot is going to be kicking out forward, your front foot is going to be pushing backwards, and you really want to guide your foot to land directly over the bolts once you're done with the rotation. If you're having trouble with this trick, you might want to try tossing your shoulders a little bit more, taking a little bit more time, trying it a little bit more slowly on even flat ground. And that's how you do a powerslide to bigspin.