To hold a Basketball with one hand you will need your hand to be at a decent size ( for basketball that is ).
Next grab the ball the same way as you will when you are preparing to shoot the ball. Extend your finger tips ( as these are what's going to help you out with holding the ball with one hand )
And that's about it. A Pictorial Explanation Would Help More But I Do Not Have The Pictures . Google the triangle finger shape during preparation for a shot in basketball technique... It is exactly the picture reference that you need