How to crack a WPA/WPA2 PSK network ?

You must sniff packets (by putting your wireless network card in monitor mode) until a handshake takes place between a wireless client and the access point. Alternatively (if your network card supports packet injection) you can try to force the cilent to reauthenticate using a deauth attack with a tool like aireplay-ng.

Once you've successfully captured a 4-way handshake, you'd need to apply a brute force dictinary attack, using a tool like aircrack-ng.

A strong WPA/WPA2 passphrase can take a very long time to crack, that is why using strong passwords is imperative.

Note that the capture phase unlike WEP cracking only needs very few frames, a 4-way handshake. What makes cracking WPA/WPA2-PSK much harder is the necessary brute-force dictionary attack.