An ER diagram looks at the entities in the clinic and how they are related to each other. So to draw an ER diagram for a clinic first you must identify the various entities such as the physicians, nurses, patients, drugs, etc then you will have to establish the relation ships between them. Eg. a physicians examin patients, nurses administer drugs and so on. you will then be required to employ the appropriate entity relation representations of the both the entity and the relation by use of any uml tool e.g. crows foot diagram.