How are these two paragraphs? Any error?

I've highlighted some of the more glaring errors.  Hope this helps. 

         This chapter most concentrates on how to write effective for public relations. Public relations practitioners have to create effective communication with the audiences. They have to understand an institution or company (unclear -- you mean the one they are working for?) and be able to analyze different sources of information. The messages that public relations practitioners want to communicate with the audiences have to be understandable for identified audiences.       

       There are two writing styles for public relations practitioners. Those are internal writing and external writing. Internal writing is for all publications that are written for a company or close to the company (what do you mean?). These publications are usually read by people who work in the company or close to the company such as the employees. These publications are corporate magazine, annual report and newsletter (try plural).  External writing or news releases are written for the public. External writing usually takes longer time than internal writing. Public relation practitioners have to have accurate information, so that the new releases will be thrust worthy. Sometimes, they will find reporting and interviewing are important for writing news release.