Mr Tall has written a very thorough series of articles about applying for a local Hong Kong Primary School for your child. They are written with the non-Hong-Kong-native parent in mind, but they should be helpful to any parent that's starting to get ready for this.
NB 'Primary School' in these articles refers to a local school, rather than an international school. Apart from a tiny number of exceptions, children entering a local primary school will be able to speak chinese fluently, and already know how to read and write a small number of chinese characters. If you are an expat parent whose children don't speak any chinese, these articles won't apply to you. You should be looking at international primary schools instead.
The full set of articles is listed on the left of your screen. They are written in the order that the key events happen in practice - so that's probably the easiest way to read them.
Feel free to leave comments on this or any of the articles if you have any questions, corrections, or extra information to add.