How can I apply a visit visa for my mum from Nepal to visit Hong Kong?

You are in HK for a visit or you have a HK ID card?
Generally speaking, you need to hand in application form with recent taken photo, photocopy of your mother's valid passport, photocopy of her bank statement or any proofs for her financial situations, photocopy of her round flight tickets and hotel booking confirmation letters in China.

If you sponsor her, you also needs to provide your application form, your passport with latest arrival stamp or extension of stay label.

If you are in China, you need to mail all these documents to the Immigration Department in Wan Chai area.

I am a hongkong permanent card holder and been working here for 9 years.

If so, you can provide photocopy of your permanent card and your passport with your latest arrival stamp/extension of stay label to apply for your mother.