I know the key blade itself and the immobiliser chip are separate issues.

I have a secondhand remote fob, same part number etc as the existing one. How do I code the used remote fob to my car? I’m told it can be done in VCDS but I’m nervous about messing it up, any help appreciated.

I'd be very surprised if a 2nd hand fob can be recoded without a new chip being soldered in at the very least.

I’ve been told elsewhere on the forum that it’s possible so I’m just checking to see if anyone knows how? 
I know a new immobilised chip is required but that’s a separate issue.

Start with an autoscan to see how your car is configured