The Outline Builder can be accessed by clicking on "SEO Tools" tab in Frase's main dashboard (not in-document) and then clicking the Outline Builder box.
The Outline Builder allows you to search short-to-medium tail keywords by typing in the field at the top left and pressing "return" (Enter) on your keyboard; you could also select any of the options in the "Autosuggest Keywords" section on the far-left panel after searching. From here, you can check off the corresponding "Related Questions" (from Google and Quora) in the next panel to add to the "Outline" section on the right.
To create a document after questions are selected, click the green "Create Document" button to create a document as shown in the screenshot below. Those selected questions are automatically pasted into that document's editor when the document is created.