You need the following packages installed:
(from Synaptic or from terminal “sudo apt-get install aricrack-ng”)
Next I will give you the terminal commands – you just have to change the parts so that they reflect your situation (for example mac addresses, interfaces etc.).
Start airmon on your wireless interface (“ip -d addr” might help if you’re not sure):
airmon-ng start wlan0
Monitoring traffic (enter number from 0 on instead of – sometimes you’ll have to backtrack here to change the channel):