How do I fill out the TDAC for a flight from Australia to Bangkok via Singapore?

To fill out the TDAC (Thailand Declaration Form) correctly, you should enter the flight number of the last leg of your journey, which in this case is from Singapore to Bangkok. Additionally, for the country of departure, you should list Australia as that is where your journey originated, even though you have a layover in Singapore. This follows the consensus from various responses indicating that the form requires the last flight's number and will recognize Australia as your country of departure.