How do I fund my TD Ameritrade account? And do I need to convert to USD first or do the conversion there?

If you transfer to the DBS account, you can only deposit in SGD. They will perform the conversion to USD and the funds will appear in your account within 2-3 working days.

If you would like to transfer USD directly, you can only do so through a wire transfer to their Wells Fargo Bank account in the US. Alternatively, you can mail them a check to their SG office in either USD or SGD but not many have tested this out and it is unclear if they will levy any charges or fees.