How do you adapt a trumpet mouthpiece to cornet?

It is not possible to adapt a trumpet mouthpiece to fit a cornet. Both instruments require mouthpieces that are specifically designed for their respective use. Trumpet mouthpieces are generally narrower than cornet mouthpieces, and they have a shallower cup. Cornet mouthpieces are wider and have a deeper cup, which makes them better suited for the lower register of the instrument.