First off, you should check to make sure you have females. They are rarely imported and most of the time only males are imported.
They are similar to breeding badis badis so all you need to do is stick your dominant male in a large enough tank with a bunch of females. Add plenty of moss, weedy plants, and hiding spaces to break the line of sight. Condition the darios with feedings of live and frozen foods.
Sometimes the least dominant male also is mistaken for a female. Just to be sure remember that if all else fails look at the behavior. Between dominant male and the others. A simple way to know is that max size till record for dario dario male are max 0.75“ and females 0.51“. So the size significantly does vary if you look closely.