How can one invest in bonds?

The idea that bonds are considered a safer investment because their price movement fluctuate less drastically (That also depend on what bonds you buy)

The general belief is that when stock market do badly, bond market will do well & vice versa.

Personally i treat the amount of CPF SA that i hold as my Bond allocation of my investment portfolio.

You need a brokerage account and an idea what you are looking for as to bonds. there are many different kinds of bond, some much riskier than others (generally higher yield is correlated with higher risks)