Login in your Account Portal, Click on Phone Numbers. Select the number you want to configure to set up call forwarding..
Click on any number and select "Edit Configuration" to go to configuration screen.
Select Forward to Phone Number and enter the 11 digit valid Phone number to which the calls should be forwarded.
Click on Save.
This is how the configuration will look after applying the Call Forwarding.
There are lot or advanced options, Click on the "?" to see the description for each option.