You don't, generally. In circumstances it is acceptable to post a question for the sole purpose of answering it, but doing so should not be taken lightly, in my opinion.
For instance, if it's a common problem and there's demand, then there could be a call for it. You have to be careful not to duplicate content though, even (or particularly) questions; if it is a common problem that people often need an answer to then the chances are your question already exists and you only need find it in order to answer.
An example of posting a question and answer might be to give a definitive, canonical pair. For instance, a great many people ask about NullReferenceExceptions in .NET, each in their own way. often not even mentioning the exception name - the answer is always the same, just a variation; now, it could make sense to create a pair to explain this thoroughly, that anyone with the same problem will be able to find using terminology the compiler gave them, and understand that the operation is the same, half of 5 is the same thing as half of 5 dollars or half of 5 pounds.
John Saunders did give such a pair.