When you misspell something, you spell it worng. Whoops. Sorry about that — When you misspell something, you spell it wrong.
When you have the letters in the wrong order or you don't have the right letters, you misspell. How humiliating it would be to misspell this word! Just remember that you need two s's — the one in mis- (as in mistake or mishandle), meaning to do something the wrong way, and the s in spell.