How we draw a usecase diagram from a case study?

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Main pointers to be remembered while drawing a use case diagram are:
1. Have one round of overview on the case study and come up with the list of actors who will have interaction with the system.
2. For these actors, system should have use cases, and use cases are always "actions" that actors interact with the system.
3. So, taking point-2 as reference, break the case study into smaller chunks and try to analyze every point. Identify if they have interaction with the system or not.
4. If they have, notate them in verbal forms (as an action, Ex: Makes Payment, Generates Bill). These can be termed as 'use cases.'
5. Map them with the corresponding actors.
6. Try to identify include, extend and generalization relationships, if possible as they add more information to the diagram.
      a. include : When your child use case cannot exist without the parent use case, they have an <include> relationship established.
      b. extends : When your child use case can exist with/without the parent use case existence, they have <extends> relationship
                         established.
      c. Generalization : When the child use case exhibits the properties of a parent use case, then they can be exhibited by
                         generalization relation. It implies for actors also. Ex: When financial transaction is a parent use case, Pay by Credit
                         Card, Pay by Cash will fall as child use cases. Pay by Credit Card, Pay by Cash are type of financial transaction.

These are the major concerns to be considered while deriving a UCD from a case study. Hope it helps !!!!

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identify the Actors (role of users) of the system.
For each category of users, identify all roles played by the users relevant to the system.
Identify what are the users required the system to be performed to achieve these goals.
Create use cases for every goal.
Structure the use cases.
Prioritize, review, estimate and validate the users.

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A usecase diagram (UCD) basically models how external actors are interacting with the system. UCDs depict 4 major elements:
-The actors that the system we are describing interacts with. actors stay away from system boundary and reusable actors are placed towards right side of system boundary.
-The system itself.
-The use cases or services that the system knows how to perform. use cases are verbs and are unique.Essential use cases make sense and completeness to the end user.
-The lines that represent relationships between these elements.

Few steps which we follow to draw a usecase diagram from a case study are:-
1.Information which we do NOT model in Use Case diagrams are   
 Names of the systems (laptops, Desktops, Workstations),  Architectures (2 Tier, 3 Tier, n Tier, Client Server),Networks (LAN, WAN, Internet), Brand Names (HP, Lenova, Wipro, Sony),Technology Names (Java, .Net, Mainframes) Differentiate information
2.Differentiate information against Actions
3.Write all sequence of Actions
4.Try to find out which actor is performing the above action
5.Try to identify Essential Use cases and Supporting Use Cases
6.Try to identify some modules with respect to functionality or usage.
7.Try to draw the relationships appropriately between the identified Actors and Use cases.
8. We should not model Database of the system as a separate actor.

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Steps to follow to draw a use case diagram:

1)   First find the actors i.e.; how many actors are there.
2)   Find the essential and supporting use cases.
3)   Find which actor is performing which action.
4)   Draw correct relationship between the actors and use cases with their essential and supporting use cases.
5)   The information that we do not model while drawing a use case diagram are:
   a)   Names of the systems
   b)   Architectures
   c)   Databases
   d)   Networks
   e)   Technology
   f)   Brand names