That's weird, I just found a similar thread from 4 years ago where you were discussing the same thing.
Has been done so often, search MK4 golf double din on Google for lots of images and install videos.
You need an ISO wiring adapter, trim surround and possibly an aerial adapter.
The head unit will likely come with a double din mounting frame, if not, you need one of those.
Check out autoleads, connects2 for parts. Think Halfords might even have a section on install adapters.