See the attached video for a step by step view of the process.

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 enable.  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.