How can I form the triangle at the top of booty after BBL?

Without examining you or seeing photos its difficult to give specific advice but it sounds like you may need a bit of touch up liposuction to the area to help flatten it out. I'd suggest booking an appointment with your surgeon to be examined and see what can be done. Best of luck! 

Hello,Thank you for sharing the information, it is a pleasure for me to answer your question.It is difficult to evaluate the options without seeing photos,
It is advisable to consult with a certified plastic surgeon, for a goodevaluation, to provide you with all the information you need to know about the appropriate recommendations, so that you can achieve the desired goals in your body.
I hope this answer will help youMy best wishes for you...