Male and female bettas cannot be thrown into a single tank and be expected to breed without major injury or death to either fish. They must be "Conditioned" before hand to increase the chance of safely pairing them. Conditioning involves slowly introducing both bettas, getting each in the "mood" for breeding, and feeding specific foods to make sure the bettas are healthy.
In my opinion, one of the largest reasons new betta breeders fail is because they did not condition their fish properly.
Food consumption is highly important during the conditioning process. It helps them to build up strength, and the female to produce eggs. I like to feed as much live food as I can during this time as I can as I find that it helps the bettas feel more "feral". These are my favorite foods to feed my conditioning bettas in order of most to least favorite.
Live Black Worms
Live Mosquito Larvae
Live Scuds
Frozen Blood Worms
Frozen Adult Brine Shrimp
Bug Bites dry foods
I feed my bettas 2-3x a day during this process.
There are many ways to condition bettas during this process. Some people will do so in their tanks, but I prefer using Jars.
I find Jars easier to keep the water conditions clean during conditioning. I do 100% water changes every other day, and syphon out poop, detritus, and any leftover foods with a turkey baster to keep the water extra clean. It is also easier to monitor their physical condition of lots of bettas in the jars.
Some bettas need to be taught to flare and keeping them in jars helps with this! As the bettas are isolated they begin to feel territorial over their space. Once the visual blocker cards are removed, a betta will start to flare at the others around them. I go into more detail about this below. On the flip side, if I want to breed in a tank that a betta is already residing in, jarring helps remove any territorial feelings they may have about a space before being reintroduced to it.
Jars require extra water changes to prevent them from cycling. I keep them heated with Seedling Heat Mats, but they can also be placed in a heated container with water. I place Visual Blocker Cards between each betta so they get peace, then allow them to see each other a few times a day. Read more about Jarring Care Here.
I don't like conditioning bettas in their tanks because often times I will use their tanks for breeding. I don't want the male feeling extra territorial of this space and to defend it from the female if he has already claimed it as his own. That being said, if I have a somewhat difficult male who isn't feeling the jarring, having them in their established tank can sometimes help with the breeding process. This should be done with caution.
Flare training can be done by keeping a betta isolated for a few days, then showing them another betta, mirror, or flare stick for a few minutes. Then go back to isolation. This may take several days or even weeks to build up.
Even female bettas should be flare trained. Here is a feisty female and her beard!
This is dependent on where your betta came from, their natural instincts, and age. If the betta is possibly under 6 months old then I recommend conditioning them until they are that age. If they are 6 months or older, then 2 weeks is the typical time to train them. Sometimes if they come preconditioned you'll only need to wait a few days until they are ready to be bred.
Some bettas haven't been flare trained and need to learn, or just don't have the strong urge to reproduce. If introducing them to a mirror over several days does not work, start by completely isolating them in a smaller tank or jar for a week. Feed live food, keep the water pristine, and give them time to feel secure in their space. Once the week is up introduce another betta next to their tank, or a mirror. They should feel more territorial and get the urge to flare. Do so only for a few minutes before removing. Repeat a few times a day until they are reliably flaring back. Give them something to build a nest under at this time.
If they still don't react after several days of trying, isolate again for 2 weeks and try again. If they still aren't flaring, they may be sick or don't have the instincts to breed. I would not breed this betta as that trait can be inherited with the fry.
Example: I spent 2 months trying over and over again to condition this male until he finally felt like breeding. I swapped out partners with a vigorous female who basically bullied him into breeding. He made a lackluster nest and did not help during the spawning process. Afterwards he ate all of the eggs. Some bettas don't want to be dads and frankly shouldn't be bred. :(
Visual Signs - He should be brightly colored, have a healthy weight, and be in excellent physical condition.
Behavior Signs - He should be lively, flare easily, and posture when presented with another betta.
Bubble Nest - While a bubble nest isn't completely necessary before breeding, I find it to be an clear tell that a male is enthusiastic and healthy.
Too Timid or Sickly - When presented to another betta while in a cup or other tank, he hides, sinks to the bottom and breathes heavily, or darts around the tank frantically. Loss of color or tail strength. Floats to the bottom in an unnatural way.
Conditioning a female betta is equally as important as conditioning the males. An unprepared female is at risk for injury, illness, or death. Female bettas will look and behave certain ways to tell if they are ready for breeding.
Visual Signs - She should be a healthy weight, have bright colors, and be visibly eggy. The space behind her abdomen (Ovaries) should be swollen and noticeable. Her Ovipositor should also be highly visible or enlarged. Some females will display Breeding Stripes (Vertical striping coloration)
Behavior Sings - We want to see them interested and engaged with the male. When presented through a cup or tank, she should either flare or try to swim towards him. The female should "bow" her head or swim to the bottom of the tank under him to show she doesn't want to fight.
Visual Signs - She is dull in color or grey. She displays stress stripes (Horizontal Striping) or is thin or has a frail frame.
Too Timid or Sickly - When presented to the male while in a cup or other tank, the female hides, sinks to the bottom and breathes heavily, or darts around the tank frantically. Loss of color or tail strength. Floats to the bottom in an unnatural way.
Too Aggressive - The female tries to fight or bite the male, and does not submit. She attacks him. Sometimes the female will give up after the male fights back, but if she doesn't she might be too aggressive for him or all males.
Betta is eggy, has enlarged ovipositor
Betta is ready to be bred.
Above Image by Wicked Bettas
Examples of Breeding Stripes.
Betta is ready for breeding.
Above is an example of Stress/Baby Stripes.
This betta should not be bred.
Once you start pairing bettas and see their personalities in the tank, experience will take over if it feels right.
Often times it looks like the male is being too aggressive and will kill the female, but he is just telling her that he isn't ready to breed and she needs to stay away from his nest. This is why it's so important to give the female a good hide that she can watch him from.
Sometimes the female will not be impressed with the nest and will hide herself until he makes it better.
You may see a female betta being timid or aggressive with a male. Sometimes they need a more gentle or forward male that pairs better with them. Give them time to try, but if it feels wrong try giving them a partner that matches their personality. For example, a more timid female may need a gentle male, or an aggressive female needs a stronger male.