Application Procedure and Requirement for Mainland Application for One Way Permit

My friend is a mainland national and holding a Chinese Passport. She is married to a British Passport Holder who has Permanent Residence in HK. They got married in Hong Kong and now has a baby who was also born in Hong Kong and holding a British Passport.

She now wants to apply for a visa to join her husband and live permanently in HK with her husband and daughter.

What document requirements are needed and what is the procedure for application?

Thank you for your help and advise.