Most indians do not buy the stuff because it looses so much flavor (much like fresh ground pepper vs packaged ground pepper). instead, what i do is to buy raw cumin (ask them to sell you good quality "zeera" if you go to an indian store). to roast them, in a heavy skillet/frying pan, put in a handful (crushing them slightly by rubbing them between the palms of your two hands) and then over med-high heat roast them dry, shaking the skillet a lot, until they are dark brown in color (but not black, over
For some dishes, like raita (raita = slightly whipped yougurt + (favorite veges, like cukes, onions, tomatoes etc) + roasted cumin powder + salt + pepper) you need to powder it. what i do is to put the roasted cumin on my cutting board, and with a roller pin go over it, and they powder with almost no effort.