How do you make sure cross pollination happens?

It takes time but you can use a Q-tip. twirl the Q-tip inside the flower of the plant to gather pollen. Go to the other plant and twirl the same Q-tip in a different flower and keep doing this back and forth between plants and different flowers each time. When I do this with my Apple trees I pull a petal off the flowers already done so I can keep track of those done and those still needing pollination.