In order to do a fakie 180 backside, which some people call a fakie half-cab, you're going to want to a know how to roll in fakie for starters. Once you're able to do that, you should really know how to do a 180 ollie, or what some people call a half-cab. You're going to want to pop your board as if you're doing a fakie ollie, except when you do it, at the peak of your ollie, you going to want to rotate your shoulders and land the trick. Some people like to start rotating before they even lift off the ground, but that's up to you. I, personally, like to get to the height of my ollie and rotate just towards the end, letting all four wheels hit the ground.
Now it's a little difficult to do this trick because, for starters, you're going in fakie, and, secondarily, if your wheels hit the ground and not all of them have contact at the same time, you're likely to fall either towards your nose or towards the tail. So, make sure that all four wheels hit the ground, and you get a nice, smooth landing. It also adds style points. That way, when you're doing it outside and your friends take notice of this trick, they'll know that you really spent enough time and effort on this trick to land it the way that you did. And that's how you do a fakie 180 backside.