The best method of teaching an urban slum about ORS is –
Demonstration 

A demonstration is a carefully prepared presentation to show how to perform a skill or procedure.
Demonstration as a means of communication has been found to have a high educational value in programmes like-


Environmental sanitation → installation of a hand pump, construction of a sanitary latrine.
Mother and child health → demonstration of oral rehydration technique,
Control of disease, e.g. scabies.
Lecture
Role play
Demonstration
Flash cards