That’s very simple!
First, you need a wallet. Second, you synchronize your wallet. Third, you wait for your coins to mature. Finally, if you encrypted your wallet, you unlock it for staking.
Once all those steps are done, your coins will automatically stake for as long as you leave the wallet running and connected to the internet.
See the question “How do I turn on staking?” if your wallet’s not staking when you expect it to.