First of all you need an SSH client. In Linux and OSX you van use the standard terminal app but in Windows you need a tool like Putty.
ssh username@<yourIP> (standard username is admin)
The IP address of your cloudgate depends on the configuration. Out of the box, this is 192.168.1.1 when your CloudGate is connected to your PC using the main ethernet port.
When using the ethernet switch card, this is 192.168.4.1.
The SSH credentials are the same as your GUI credentials.
When you see the CGate logo, you have successfully logged in! Now you can run commands like ifconfig or browse through the filesystem.