You can draw inspiration from an existing operating system or come up with something of your own.
What kind of apps does your phone have? Design the icons on the homescreen.
Draw your own background image.
Design an application on the app screen. You can use the elements from the attachment.
What happens when someone calls your phone? Design the view
How does your phone take pictures? How do you view the pictures?
What kind of a keyboard does your phone have? Does it have emoji?