Earn enough so you can invest $300 a month into the S&P 500 index comfortably. Do it for 40 years, from 25 to 65. This should grow to $932,000 at the age of 65.
Buy a MediShield Integrated Shield Plan. Buy term insurance for death, for $1 million, with a critical illness rider. Buy disability income insurance if you have the spare cash.
Before the age of 65, put enough funds into your CPF Retirement account (RA) to reach the Enhanced Retirement Sum (ERS). Estimated sum at 2050 is $659,000. Cash needed to top up depends on lifetime earnings, but believe most should have about $300,000 – $500,000 in cash left after topping up.
That’s it. Total passive income for the rest of your life is about $2,000 (CPF Life) + $2,000* (your own investments). = $4,000.

Explanatory notes

* $2,000 = $400, 000 * 6%. Assumed return