You can consult the following:

http://www.labour.gov.hk/eng/plan/FAQ.htm

http://www.labour.gov.hk/eng/public/fdh.htm

There are sample documents provided for reference on writing a termination letter and producing a document evidencing receipt of payment (settlement of any and all outstanding payments).

Immigration Department requires the employer notify the FDH section in writing within 7 days of terminating the helper.

Things to consider are how long you employed your helper as she may be entitled to:
- annual leave
- sick leave
- statutory holiday/assigned day off (if working on such days)
- food allowance (if applicable)
- travel allowance
- return ticket (either money or photocopy of actual booking)
- any medical expenses incurred by your helper (employer is required to pay for any and all medical treatments; hence FDH insurance)
- severance pay (more than 24 months employment)
- long service pay (more than 5 years employment)

Should have at least three copies (employer;helper;Immigration Department) signed and dated by both employer and helper.

Just in case, the sample documents can be found in the appendix section of "Practical Guide for Employment of FDH"
http://www.labour.gov.hk/eng/public/wcp/FDHguide.pdf

Termination Letter (sample) = Page 33

Receipt for Payments (sample) = Page 34