One of the most important things to remember when breeding beta fish is to always keep males away from the females.
You really only want the male and female Bettas to mingle together when you're considering breeding beta fish. Most breeders will only leave a male and female together untill the job is completed.
And you should expect that both Bettas will suffer some wear and tear in the mating process.
Check The Size Of The Female
If you're dealing with much younger fish, female Bettas should be somewhere close to the male's body size or at least three quaters of the size. Usually the best way is to put a young male in with an older females which is a bit safer for the female Betta.
Another thing to keep in mind is that breeding beta fish isn't an easy task. And don't be dismayed if things get pretty rough durin the mating process.
How Bettas Get Damaged Fins
Oftentimes before the Bettas actually do mate... a male and female will fight. Sometimes to their death. But NOT often. Which is probably not what you would expect during the mating process.
Sometimes a male Betta and female Betta won't be able to mate at all. This is usually the case when a female can't or won't flare back at the male and/or trys to hide in a corner of the tank. If this happens you should try again with them at a later time.
Full Contact Mating
There's not much sense in stressing the female betta and risking her life unnecessarily if she isn't ready for the mating process.
You should keep in mind that with siamese fighting fish (aka., Betta fish) breeding is a full contact sport. Once a male wraps himself tightly around a female to mate, oftentimes she may appear to look somewhat stunned in appearance.
This is quite normal...
Females can also lose some of their scales and even wind-up with frayed fins during the breeding process.
As soon as a female releases her young eggs the male usually takes over. When this happens it's time to isolate and remove the male Betta from the tank because a male tends to get hostile towards the female and will oftentimes kill her.