Chinese national with US green card wants to travel to SK from China. What process should he use?

Generally speaking, you need to go to the nearby consulate-general for making appointment for applying or go to the appointed tour agency for apply. It is suggested to do it through tour agency. You need to prepare an application form, an invitation letter, bank statement/proof of employment and police clearance. The processing days are around 3 to 5 days based on the type you apply.

For detail, you need to go to the consulate for consulting directly.