OK! This is a fun one! You should be able to do frontside noserolls before you try this, switch backside 180s would be good to know as well as being able to balance on your nose fairly well.
1. Approach with your weight forward. Fix your eyes on the point where your going to pivot. Unwind into the spin as you start to pivot.
2. Plan ahead and try to put your weight where it should be once your board has spun 180 degrees. Your hips should sit directly above the wheels your manualing on when you complete the 180 degree rotation.
3. Critical point, with your weight over the front you now need to pop the board up and spin the last 180 degrees. I try to keep my eyes fixed on the landing and spin the 180 with the bottom half of my body, but that's just me! It is possible with practice to hold the board in a switch manual like this for some time before popping the switch backside 180.
4. Ok your spinning round now, keep your eyes on the landing and brace to land on the ground and be ready to make and correction needed from not landing quite straight.
5. It starts getting difficult to see your landing but you should know by now how well you're going to land it. Just as I'm about to land I'm normally looking directly where I've come from.
6. Stick the wheels on the ground and correct for any shortfall in the rotation. Now ride out and do it again!!
It took me a long time to learn this one, its just one of those tricks which needs a lot of practice. Once you have got it down there are a few variations, you can jump the 180 into the manual before jumping 180 out again. This allows you to jump over small objects or onto kerbs etc. You can throw these pretty much anywhere, in the forest, on the service roads, on boxes, or in the street.