Here is a net run-rate calculator for group 2 of the World Twenty 20.
There are two possible scenarios where net run rates (nrr) will come into play. Firstly if India and Pakistan both win then there will be a 3 way tie between India, Pakistan and Australia. Nrr will decide 1st, 2nd and who goes home in this situation. The other situation is if Australia and South Africa both win. Here Australia will go through as the top qualifier, and it will come down to nrr to decide which of South Africa, Pakistan and India go through.
There are some instructions inside the frame, but you may need to scroll around. Once you type in some number the net run rate will automatically change on the right. Again you might need to scroll to see it.