One of my favorite tricks of all time, and one of the few tricks that came natural to me from the get go. I think I learned them in 5-10 minutes my first time ever trying to actually learn them. I suck at giving trick tips, but I'll try to explain it the best I can.
Have your front foot wherever it feels comfortable, it's not going to be as important as your back foot. Some people like to dangle some toe off like a similar front foot setup to a heelflip, but I have my front foot on there with nothing hanging off, kinda like how you would set your front foot up to just do a normal ollie, but just do what's comfortable to you.
For your back foot, you're gonna have your toes in the left pocket of the tail (right if you're goofy) but don't have it be JUST your toes, have a bit of foot on there too, but it's gonna be mainly your toes. With your back foot you're gonna scoop outward with a lot of might, and let the board rotate and guide itself to your front foot. You might wanna practice the scooping motion for a bit to make sure it gets all the way around. Get used to the opposite motions your feet are going to be doing (back foot is kicking outward, front foot is moving the opposite direction to catch the board and stop it from rotating). When I first learned it I was picturing it as an fs 180 except my body stayed still while the board rotated, and that kinda helped, but for some people that tip doesn't work for most people. Another important tip: Have your body, more importantly your shoulders and your whole upper body in general, facing forward, as if you were getting ready to do a Fs 180 (like how Reynolds describes in his Fs flip trick tip on YouTube, shoulders and upper body facing forward as if you're about to turn even though you aren't going to). You will have more control of them and they will feel better (for me at least) when you do them like that, and it'll be easier for them to rotate all the way around, so if you're having trouble with a full rotation, try this method out. I've always found them harder to do when you're not facing forward but just facing to the side like you would be for kickflips and a lot of other tricks.
Also, to the poster above me, I hate when that shit happens, when you learn a trick and they make so much sense for a short period of time, but then you immediately lose them not too long later after not doing them for a bit. Happened to me with regular bs bigspins. Had them down so good for a bit, stopped trying them for a while, and now I can't do them anymore at all.