You should be comfortable with riding your skateboard before trying this trick and you have to be able to do a frontside 180 before trying.
This trick relies heavily on balance and should not be hard to learn. Basically, stand in the same stance you would for a frontside 180, but when your in the air try to balance yourself so your weight is overtop of your switch stance's back foot.
When you are doing this, focus a lot on your balance in the air, and where your weight it. You want to put the wieght in the right spot, and when you are going to land make sure you have your back foot (of your switch stance) bent so you can absorb the impact and land on doing the switch manual. It may be a good idea to have your front foot far up the board (in your switch stance) once you land, doing this will make landing in the switch manual easier because you have more control of your board.