If you have a male and a female, there is no "make" to breeding. It comes naturally and they will breed when they get old enough to do so. But, if you have two red ear sliders, I hope you'll reconsider and separate them. Red ear sliders are like feral cats. There are too many of them in rescues across America looking for homes. Some rescues won't even take them any more because its hard to find homes for them.
They can't breed until sexual maturity, which for most torts/turtles will be at a certain size not a specific age most times (varied based on breed). Please reconsider breeding any tort or turtle. There are things to think about such as: finding homes for all of the potential hatchlings before they come into the world (or being prepared to keep them all), costs associated with eggs like the incubator, the stress or health problems on the female, and the sheer amount of some breeds that need homes such to not bring more into the world.
My coworker bred her cats for it to be an experience for her children. I think she did it right though, because she had a list of people lined up for the kittens before they were born, had thought about this for a year, and planned to only do it once and got them fixed right after. Just an example.