There's a couple of different DGS filter / gearbox types, and the oil is about £20 per ltr as it's specific stuff. Basically, if you can get your dealer to do it for £170 or so I'd not bother getting dirty. From memory it involves dropping the oil out, removing the airbox and removing the filter which is sited underneath it, replacing the filter, filling with oil until it overflows. Connect VCDS and run the engine up until the gearbox fluid hits a specific temperature while the excess runs out of the overflow/drain hole, then refit the sump drain plug. All a bit of a chew on really.