How do you write a use case?

First we have to read the case study properly. Find all the actors and differentiate the information against each actors. Make a sequence of all actors. Then we have to find out the actions performed by each actors from case study. Then try to identify essential use case and supporting use case. Identify some modules with respect to functionality or usage. Then we have to dry the relationship appropriately between the identified actors and use case. We have to identify the information which we don’t model in use case, like name of the system, networks, technology names, brands name.