In case you are overseas, you can always call for an emergency assistance. And in order to claim for a travel insurance online you can call the hotline number of DBS Bank any time you wish within a month upon your return to Singapore. You can also download the claim form from their website.
the process depends on the company where you buy the insurance from. Like others had said, call their hotline. You should always keep a copy of your insurance policy in your email and print a hardcopy to bring with you on your trip. There, you will find the hotline number.