I won't address why you need index funds again.Going straight to the point, there are 3 ways
1. POSB $100 min a month investment in Singapore Equity and Bond ETF. Search POSB ETFs and you will see a monthly investment plan. There is a sales charge apart from the monthly fee.
2. You can buy ETFs off the exchange. To do this, you need an account with a local broker. You will always need to pay the minimum transaction fee of $25++ per trip.
3. You can buy ETFs off the exchange with Standard Chartered Bank. Note that they will be your custodian, not the CDP. There is no minimum transaction here.
Other ways - you can buy from Fundsupermart, but I am a strong believe of going direct to buy from the exchange, never from an intermediary.