Sun dry everything for a day or two. Since this powder is stored for months its necessary to ensure that all the ingredients are properly dried.
In a heavy bottomed pan (preferably hindalium), dry roast Coriander in low flame for about 5-7 min until its fragrance arouses. Make sure to roast coriander in batches if you are opting for large quantities. After roasting coriander, switch off the flame and add cumin and pepper to it. Mix it some time or else bottom layer might get burnt.
After a few minutes, transfer to a plate and cool it down.
Keep red chillies ready by just sun-drying it. No need to roast. I have used ¾ red chilli and ¼ byadagi chilli as the latter gives bright red colour, keeping spice level to minimum.
Mix everything together and grind it in a mill. If there is no provision of mill nearby, prepare all the ingredients in minimum quantities and grind in a mixie. Store it for nearly an year in air tight container.