I am a US Green Card holder and a Pakistani citizen. I am planning to go Pakistan via the UK, staying for 5-6 days to visit my cousins. Which visa do I need and what is the process?
I am assuming you want to stay 5 or 6 days in the UK on your way to Pakistan.
You will need a standard visitor visa to spend that long in the UK. This is the government website explaining how to apply for it.