Paperwork can take up to 8 weeks so I would recommend you start looking 3-4 months before baby is due. If you hire someone who is finished contract processing can be done faster and you can apply for change of employers 1 month before she leaves her previous employer. However the processing takes longer for helpers who break their previous contract. Broken contract helpers will need to exit HK whilst their papers are being processed.
Spread the word amongst friends/coworkers and I would also place an ad on GeoExpat.Com. Try and include as much details in your ad. Here are some tips on finding the right person
Would recommend making a list of what you're looking for in a helper: cooking skills, experience with babies/toddler or older kids, good cleaner, good communicator, good references (do call up previous employers).
As you scan through the Geoexpat ads, you can shortlist the candidates. Then text each one to ask them to call you for a phone interview.
Have some simple questions prepared for the phone interview. If you have trouble understanding her or she you, move on to the next person. As a working Mum I have to give a lot of instructions over the phone, and we need to be able to communicate.
If she answers the simple questions well, move on to some more complex questions. If she enjoys cooking, ask what her best dish is. Or if she takes care of the kids, what games she plays. If the phone interview goes well, ask her to meet you for a face-to-face interview.
Most helpers are free on Sunday or Sat and will come to your home. Line up a few interviews on Sunday. If you have kids, make sure they're around and see how they interact with each other.
Also you can check the helper's passport to see who her previous employers are. Just to verify her work experience and the number of yrs she's been in HK.
Do call previous employers for a reference. If they have worked for Chinese families, who can't speak English ask a friend or colleague to call for you.
Also have a read through this article
Hiring A Helper: Interview Tips - Hong Kong GeoBaby
Place an ad on GeoExpat here:
https://hongkong.geoexpat.com/classifieds/helpers/helper-wanted