What size is your tank? Sororities don't work in anything less than a 10g because you need at least five girls. I have no specific advice because one method that works for someone may not work for someone else. All I can say is don't give up. It may take several tries at introduction before they finally stop trying to tear each other apart. Watch carefully to see who is the aggressor and who is getting beat up the most. When you have to reintroduce them, and you will, put the girl who is getting picked on in the tank first and the aggressor in last. You will know when the fighting is too much when you don't see any girls backing down from a fight. What you want to see is chasing and nipping, not a knockout brawl.
Hopefully someone will come along with some solid advice for you. Good luck.