you can make it in person to the Extension Section or any Immigration Branch Offices(except Hong Kong Island Travel Documents Issuing Office)before or within seven days from expiry of the limit of stay. But applications will be considered on its own merits. The documetns you need to bring there are your travel document; evidence to support the need to prolong the stay in Hong Kong e.g. onward/return tickets; evidence showing adequate fund etc; and completed form ID91.