How can I stretch my buttocks or make the skin loose to achieve a BBL?

You really don't want to stretch the skin before your BBL.  The fat injection itself should stretch the skin appropriately.  

Thank you for your question. Did your plastic surgeon tell you it was necessary for your skin to loosen up before your BBL surgery? This is not something I have heard before. The only option to "potentially" loosen skin is to lose significant weight, but even then there would be know way of knowing if the affects would impact your buttocks. More information would be helpful. 

In most cases, slightly tighter skin is an advantage, not a disadvantage, when performing a BBL.   With a tighter skin envelope you will achieve a better (tighter and higher) result for the same amount of fat transferred,  than if the skin were looser.  Check back with your surgeon to clarify their recommendations.  Best wishes, Dr. Lepore.