This dish is generally served over rice, so I recommend that as you start making this dish, you also start some rice in the rice cooker. That way, you'll have a hearty plate of rajma & rice ready in about 15 minutes.
Now add the salt, mustard seeds, cumin seeds, and cayenne pepper. Stir around to prevent burning. This step is done to bring out the flavor of the spices. Many Indian spices (and probably spices in general) are fat-soluble, so this "roasting" process draws their flavor into the oil and thereby spreads it evenly in the dish.
Once the mustard seeds start popping vigorously, it's time to add half of the tomatoes. If you're feeling lazy, you can add all of the tomatoes at once. The tomatoes will be cooked down into a more paste-like substance, and saving half of them for later is just used to enhance texture.
Cover the pot and let it cook for about 3 to 5 minutes. At this point, add the beans and the rest of the tomatoes. There is no need to drain or rinse the beans if you use Progresso or Goya, since there's not much liquid involved. If you have a very "wet" can of beans, you might want to get rid of some of the liquid.
Stir for a while, and let it cook for another 10 minutes or until the beans start breaking apart slightly. At this point, the dish is done and can be served over rice. This dish also refrigerates well, so extra portions can be made once and eaten later.