You need to marinate the meat overnight, or for at least six hours, in the yoghourt, garlic, ginger, half the saffron, salt, green chillies, chopped onion, tomatoes, tomato purée and half the whole masala.
Cook this in the oven in a covered pan (350°F, 180°C, gas mark 4). The meat needs to be soft. It takes about an hour and a half but may need less or more