Just like the previous Do Not Disturb feature, Apple's new Focus Mode setting can be easily accessed by pulling down the Control Centre from the top right corner of your screen. While you can only add new categories through the settings app, the Control Centre offers a long list of all your unique focus modes.
From this menu you can easily turn mode on or off by simply tapping on it. The three dots on the right of each category brings you directly to the settings app in which you can further customise your current focus categories, or create new ones.