Home Affairs Enquiry Centres administer declarations for private use before a Commissioner for Oaths, free of charge, and for everyday matters that is the obvious route. There is one trap: the Enquiry Centres require their own specified declaration form, and a self prepared document cannot be substituted. The answer is to attach your drafted declaration as an exhibit to their form, which is what the Department itself suggests. Time slots vary between centres, so telephone ahead.
Solicitors and notaries work differently. They take a declaration on your own document, which is why anything with numbered paragraphs, exhibits and a bespoke recital goes to a solicitor rather than to a counter. Bring your original identity document and sign nothing before you arrive. The firm name and address in the jurat is precisely what banks and registries look for.
Documents heading overseas need a further step. The Registrar of the High Court is the competent authority for Hong Kong under the Convention of 5 October 1961 Abolishing the Requirement of Legalisation for Foreign Public Documents, and the Apostille Service Office issues the certificate. An apostille authenticates the officer's signature, so a declaration bound for a Convention country is taken before a notary public first. An electronic route now runs through registered notaries.
The Mainland sits outside that system. Documents emanating from Hong Kong for use in Mainland China must be attested by a China Appointed Attesting Officer, a local solicitor separately authorised by the Ministry of Justice, and no apostille will substitute. Non Convention countries require consular legalisation instead.