We've shown you how to tap your network and monitor everything that's happening before. However, blogger Jacob Salmela's shows off a trick that makes it even easier using OS X Yosemite's packet sniffer.
To access the packet sniffer, open up Wireless Diagnostics (hold down the Option key, click the Wi-Fi icon in your menubar and select "Open Wireless Diagnostics"), then click Window > Packet Sniffer. Select the channel you want to monitor and click the Start button. When it's finished, you'll get a WCAP file you can open with Wireshark.
OS X Yosemite Has a Secret Packet Sniffer [Jacob Salmela]