-you need to visit dharamshala
-need to take taxi or local bus to ghera village
-then there you have to treck to lake which takes upto 10-12 hours depends upon your trecking speed.
Kareri lake is 14 km trek one side from Kareri Village.
If you are going from Delhi. Take overnight volvo till Dharamshala.
Once you reach Dharamshala you can take taxi/shared cab to Kareri Village and start your trek.
It will take around 10hr max to summit kareri lake.