How to naturally stop breastmilk?

Hi mummies, I have been exclusively expressing milk for 10 months. How do I reduce my breastmilk naturally? Now I pump 3 times a day, almost 7-8hrs apart, each session about 30mins. Each session produces around 300+ml. Tried one day to reduce to 2 pumps and my breasts hard like stone. Very uncomfortable. I just introduced formula milk and wish to stop pumping. Any recommendation to slowly reducing milk supply?