1. If there are multiple extensions on your phone, only the "first line" can call-forward. If the line you are forwarding is shared among multiple phones, only the primary phone can call-forward.
2. Click on the "Fwd All" softkey button underneath the LCD screen
3. After you hear the two quick beeps, input the number you with to forward to (example: you want to forward your number to 4158675309, you would then input: 914158675309)
Now if you look at your extension on the LCD, below that (per the example above, will show "To 914158675309"
To undo the call-forwarding, simple press on the "Fwd Off" button.
See also: How can I forward all of my Cisco desk phone calls to my mobile phone remotely?