Before you can buy SSB, you need to make sure you have a Central Depository Account (CDP) account. You can apply for a CDP account here.
DBS Internet Banking
Login to DBS Online Banking Site and follow the steps. (Note that you cannot apply for SSB on the DBS Digibank app.)
OCBC Internet Banking
Login to OCBC Online Banking Site and follow the steps.
OCBC Mobile App
Login to your OCBC Mobile App and follow the steps.
UOB Internet Banking
Login to UOB Online Banking Site and follow the steps.
UOB Mobile App
Login to your UOB App and follow the steps.