If you are using dry black chickpeas, soak them in hot water for about 4-5 hours. Pressure cook them until soft. Instead, you could use canned black chickpeas or skip chickpeas completely.
Peel and chop the potatoes to small cubes and boil them in water. Do not overcook, they should be just perfectly cooked and softened.
Take a small gooseberry sized tamarind and soak it in warm water along with jaggery powder. Squeeze out the tamarind pulp as much as you can, and make sure the jaggery is melted. Strain it and set aside.
In a mixing bowl, add in boiled potatoes, black chickpeas, salt, red chilly powder, cumin powder, garam masala, prepared tamarind-jaggery pulp. Mix everything well, check for salt and spices and add more if required. Set this aside.