I haven't got one but I like the idea of them. In theory, if you have a TDi, you shouldn't need one, because the cyclone breather that bolts to the head is effectively an oil catch tank that returns the oil directly to the sump, they don't seem to do that all the time mind! As for fitting a catch tank, it's dead simple. They come in different shapes and sizes and will usually have two or more mounting holes. You'll probably have to fabricate a bracket, then you simply need to bolt it somewhere and run a length of pipe between the breather outlet on your rocker cover (cyclone breather vacuum pipe on a TDi, or breather pipe between oil filler cap and air intake on a 12J / 19J) and the air inlet. So basically, one pipe in, one pipe out, so that the air intake draws combustion gasses through the catch tank. Most of them require you to empty them manually, rather than having a feed directly back to the sump.
-Pos