Such a script would include the causes for the rise of the Populists. So it would have to deal with the difficult conditions faced by farmers in the South and the Midwest during the late nineteenth century. It would have to discuss falling crop prices and especially the role of the railroads, who used a number of different predatory business practices to extract as much money as they could from farmers, who depended on them for shipping purposes. It would also have to include some discussion of the monetary issues that concerned the Populists, including the closely related issues of debt and currency. As debtors, the Populists wanted an expanded, deflated currency issued against silver, which placed them in conflict with business interests who favored money based on the gold standard. Finally, your evening news script may want to address some of the demands of the Populists. These included addressing some of the issues above, like unlimited coinage of silver, but also some more sweeping reforms like a progressive income tax, nationalization (or at least strong regulation) of railroads, the eight-hour workday, the secret ballot, and direct election of senators. 

How do I write an introductory news script on the Populist Movement?

I think that an introductory news script on populism is going to be well set to focus on the initial idea that the masses have come together in the name of social change.  Consider the idea of reporting on a protest or demonstration in which people from the middle and lower classes have all converged onto one location, demanding social and political change and chanting slogans in unison to accomplish their purpose and raise awareness to their cause.  The introductory news script might start to describe this setting and outline some of the causes being pursued.  From this point, I think that some level of interviewing of the Populist leaders and participants in the movement can be done.  The introductory news script has to bring the reader into the full force of Populism which is something well accomplished by being able to focus on the idea of massive protests with people from various walks of life trying to accomplish some level of substantive change.  In this, I think that the news script can introduce Populism in a manner that highlights its importance and understanding.