Easy. You buy a male and a female. And then you've got a pair. And they will breed. Sometimes in the bag on the way home, and sometimes in a day, but you shouldn't have to wait more than 6 weeks or so.
Females have orange one the sides near there belly area. Get a male and female, provide them with an adequate size tank(20g) with lots of hiding spots. The cons will take care of the rest
The pic below is of a female pink convict, but the regular barred females will have similar coloring.
Just get 2 convicts above 1" in size and put them in a 20G or larger tank with plenty of caves. Then add water and wait a week or so before you have eggs. Simple.
I hate the thing with con fry is that as soon as you put them in the tank to be fed, they just go squirming all the way to the bottom before the fish see it.