Press the center button on the cursor key pad to bring up the menu.
Press 6 or navigate to "Phone" then press select.
Scroll down to "Call features" then press select.
Choose the line you want to forward then press select.
Enter the number to forward to, then press the "Save" softkey. The number should be formatted exactly as you would dial it manually. So a 3-4 digit extension or a ten digit outside phone number. This saves your forward-to number permanently (until you change or remove it). You can now enable/disable forwarding very easily without entering the number again.
You will need to enable it now, or at any time, by repeating the above steps then pressing the "Enable" softkey, or the "Disable" key to turn it off.