The most important thing about Outlinely is, Outlinely is a text editor/writing app with an outlining engine built on top of it. So if you love text editors, you will feel right at home because there is virtually no learning curve. Just learn some outlining features and you are good to go.
A great way to learn to use Outlinely is to have a look at the list of keyboard shortcuts.
In the app, press ⌥⌘/, (or select menu item Help > Keyboard Shortcuts) to bring up the Keyboard Shortcuts Panel.