What do male bettas do when then want to mate?

If the male is not interested in the female betta, (which is usually the case when bettas are to old, too young, or the female is not ready to have kids) the male will attack it.

If the male is interested, then the male will make a bubble nest in order to keep the eggs safe. Then, the male will chase the female violently. This is called a courtship. The male will then fertilize the female's eggs and place them in the bubble nest. He will then chase the female away. He will either eat the eggs or guard them. If he guards them, eggs will continue to fall off the nest, so the male will pick them up with his mouth and put them back until the eggs hatch.