First off, to learn this trick, you obviosly need to know heelflips and B/S 180's very good. Put your front foot a little further back than you would for a regular heelflip. Put your back foot just like an ollie. If you put it like a B/S 180, you may have trouble flipping the board. Roll at a comfortable speed. Put more weight on your heels than on your toes, this helps spin the board faster. Pop pretty straight, no more than 45 degrees, to enable getting height. Keep it under you, don't dip in your head. Now, for the catch. You have 2 options now.
1. Land the trick fully rotated. For this one, you need to pop and jump higher, so you can land a full 180. You also need to try to spin it faster, to make sure you can get around.
2. Land less than 180 and pivot. I think this option is easier. You need to pop straighter, and catch it a little early. Right when you catch the board, or it hits the ground, whichever happens first, get all of the weight on your front foot, and slide around to complete the 180.
I can't flip the board all the way.
Experiment with your foot positions. Also, keep weight on your heels, and flick out very fast.
I slide out when I land.
This is all about weight distribution. Just practice a lot, and stay centered over the board. You will get it faster if you already know a 180 fliptrick.
When it comes down to it all, just practice, practice, practice! This is a really cool trick, and looks amazing down sets/gaps.