This dye permeates plasma membranes very slowly, so it does't work well in intact cells. It can be used with good success in isolated mitochondria. Otherwise you might want to choose a different dye, like TMRM for instance.
There are plenty of JC-1 kits out there for measuring mitochondrial membrane potential. Thermo Fisher, Abcam, and Cayman Chemical all have them (with protocols).