how to get a Z visa

If you want to work in HK, you need to find an employer first and then apply for a HK work visa. You and your employer should submit required documents to HK Immigration Department. If you are in South Korea, you can submit your application to the Chinese embassy there.

what documents should be provided. thanks for the information

There are two kinds of situations: employment as professionals or employment as imported workers. They have different requirements. I suggest you read this page to learn more about HK visa.
http://www.immd.gov.hk/en/services/hk-visas/index.html