Think about how you want your main character, the god or goddess, to appear. Write down hair and eye color, unique facial features, height and other striking features. You can exaggerate your own features, or invent some.
Create a family tree for your main character. Where does she come from? What other characters will appear in this tale? Draw on members from your family, and hone in on some of their unique qualities. Then picture them in a Greek myth.
Create a personality for your character. What are his interests? Is the character romantic or mean and spiteful? Think of personality traits that might be fitting to a Greek god or goddess.
Finally create the god trait. What is her area of expertise? What makes this character a myth? Try to focus on something here that really interests you. What do you think the coolest trait could be and how would that affect the world?
Using the family tree you created, go back and create the history behind your character. In Greek mythology, ancestors created the world. What did your hero's family do? Maybe he created water. Or, maybe he created the mountains and fire.
Create an issue where your character's god trait will come in handy. Create your myth around that -- something that will live on for centuries.