Offer your baby a light object — a soft ring, a wooden spoon — held out to one side so they must reach across with a specific hand.
Once it is held, offer a second object to the same hand. To take it, they have to move the first one across.
Do not move their hands for them. If the object drops, that is a real attempt; hand it back and let them try again.
Moving an object from one hand to the other means both sides of the body coordinating on a shared goal, which requires the two brain hemispheres to communicate. It is a developmental milestone in its own right and the root of every later two-handed skill.