Toast 1/4 cup each cumin, cardamom and coriander seeds in a pan until spices become fragrant and begin to pop. Cook completely, then grind in a spice grinder or coffee grinder on the finest setting. Add 1/3 cup turmeric powder, 2 tablespoons mustard powder and cayenne pepper to taste. Keep sealed in an airtight container. If desired, you can also add ground ginger or other spices you love -- in India, every curry is different, so feel free to customize yours.