How do I begin investing in the S&P 500? What's the simplest/most cost-efficient way to go about doing this?

I think Endowus has such option.

How can I invest in S&P 500 using SRS ?

2 general options (but of course more...)

#1 open free online brokerage account with TD Ameritrade or Charles Schwab, they have no commission fees for U.S. ETFs, how nice!

You could buy tickers VOO or IVV, which are large and mega-cheap

#2 open an online brokerage account with a cheap SG brokerage that also has access to european markets. ticker of choice would be VUSA, as opposed to the other option (Singapore listed SPDR, that has much higher annual total expense ratio)

You could

Open a brokerage account from any regulated broker with strong regulatory nations such as Singapore and the US, and you can then invest in any ETF or mutual fund that tracks the performance of the S&P 500.