To do a 180 you will stand in the position somewhat like the ollie, but you will have your back foot more near the back side of the board,(on top of the little groove that seperates tail and the middle of the board) then you wind up by turning your arms so that you would be looking at your back foot. Bend your knees and pop your board and drag your foot up while you unwind to do a 180 in the air. Shift your wait to your what used to be your back foot and extend your legs down and bend your knees again with the weight of your body. Keep balanced and roll away.
Tip: Practice rolling around switch to have better landing balance.