How to Betta fish mate?

It is quite an interesting process. At the bottom of this answer there is a neat, must see, video that should be looked at full-scree for best..

Normally the pair are kept apart and out of sight of each other. A breeding tank of a few gallons should be set up and the male placed in it to establish territorial traits.

A female in a seperate, smaller container is placed in the breeding tank so they have visual contact. At this stage the male will start demonstrating for the female and proceed to blow a bubble nest (like very white beer foam) in a corner.

Place the female with the male and they will appear to fight, but if she has eggs the fighting will stop and courting proceeds.

Both animals will come very close together and seem to be shivering. They then touch bodies and contort in a "U" form (like your two index fingers used to pull hands apart) about each other. That act is the male actually squeezing the eggs from her and at the same time fertilising the eggs.

You will see the eggs falling (like grains of salt, but slower) and the male will retrieve them by mouth and blowing them in the nest of bubbles. This can be repeated several times.

When laying is complete the female should be removed from the tank, leaving the male to guard the nest and aerate the eggs. If eggs get loose and fall from the nest, he will put them back.

Depending on the water temperature, hatching can be 36-72 hours after laying. When the fries start to swim freely, the male should be removed or he will eat the fingerlings.