You can just go to the Chinese embassy or consulate in your local place to apply for the visa with the following documents:
1.HK Visit/Transit Visa Application Form (ID1003A and ID1003B which can be download from www.immd.gov.hk);
2. one recent passport-photo size color photograph;
3. one photocopy of the passport page containing the personal particulars;
4. proof of the applicant's financial standing (e.g. bank statements, salary certificate issued by the company);
5. Invitation letter from your mother enclosed with her address and contacts in Hk;
6. air tickets booking, travel itinerary.