It is avisable to encrypt your wallet with a passphrase. If you do not encrypt your wallet, your coins will automataically stake as long as they are not reserved and the wallet remian online. (<Options><Main> and then check Reserve)
If your wallet is encrypted, simply select "Unlock Wallet..." from the Settings menu. You will then be prompted to enter your wallet passphrase.
Select "Unlock Wallet..." from the Settings Menu
check "For staking only" box and enter your passphrase
Staking is enabled
IMPORTANT: You must leave your wallet open and "unlocked for staking" in order to actively stake your coins!