Here's how to do it: Roll forward at a moderate speed with your back foot on the tail and your front foot in the middle of the board, as if you were about to ollie. Start by snapping down on your tail and moving your front foot towards your nose. As you're doing this, turn you back foot 180 degrees as well as your front foot.Stay centered above you board, landing on the trucks riding fakie. Both feet should be used equally in order to land a good backside 180 ollie.
A numerous amount of people have trouble with this trick. It does take a while to learn, so keep watching people do it and you'll get it sooner or later.
There are a few different variations of this trick. Some include the half-cab ( a fakie backside 180 ollie), nollie, and 360 ollies. This is a great trick to learn because you can add flip tricks to your 180's that look amazing.