How do you spell humongously?

You spelled it correctly! It is spelled: Humongous

Bear in mind that this Word is indicative of the fact that English is an evolving language - the Word did not exist several decades ago. Because of this, there is not always a hard and fast way to spell it. However, humungous rather than humongous brings up thousands more hits on a search, which would suggest that humungous is the correct way to spell it.