It's in the manual, which you can access by having the Context Help window open. Then, when you hover over the "Build Specification" tree item the Context Help will have a "Detailed Help" hyperlink. Click this to get to the help.
Short answer: Right-click on the "Build Specification". Select "New->Source Distribution". If you need an explanation of the individual panes, click on the "Help" button in the lower right. If you really want an LLB (not recommended), select "Custom" for "Packagin Option" and then the check box "Destination is LLB".