It's straight forward if you opt to do it yourself. Use a genuine VW rubber some of the repro ones will give you a bad time. Assuming you have no rust issues then make sure the glass is absolutely clean especially around the edges. Fit the rubber to the glass. Get some very strong cord (Local DIY shop sells it) and feed this into the groove which will fit over the lip of the door so that the ends pass each other. I use Waxoyl to the paint the lip of the door and the rubber, this becomes the lubricant. Get an assistant to offer up the glass from the outside.
Now comes the fun part. Get your assistant to force the glass and rubber into the apperture where the 2 cord ends cross, then pull one of them so it pulls the rubber over the lip. It helps if your assistant can keep the pressure on the glass as you progress along the rubber. I usually start in middle of the bottom run, work to one corner then go back pull the other cord until your reach the opposite corner and work your way round side by side until it's all in. Easy.