I've breed many fish over the years, and i love livebearers and not just the common types, i have had over 200 species of livebeares and breed the majority of them.
With black skirt's their not that easy, to breed and very difficualt to rear the fry.
You idearly need a "Breeding tank", keeping other fish in there will only cause problems
You need soft water, to start collecting rain water or useless your lucky to live in a soft water area.
Do good water changes every week, and check the qualitys to make sure their all good.
Once you want to rearly breed them u need plenty of small live food, vinager ell's, mirco worms, grindel worm, brine shrimp eggs, and daphina.
Fit the breeding tank with marbles covering the bottom to stop the parents from eating the eggs spawning mops as they may want to place the eggs off the bottom but they are classed as egg scatterers.
Large water changes can be a good trigger to breeding. but this is not always the case, it's just luck sometimes.
Good luck though.