Develop a concept listeners will buy. Your radio broadcast should be something believable, or it should incorporate subject matter that is a part of the general public's current interest or concerns. War of the Worlds was written at a time when a belief in, and fear of, visitors from another world was prevalent. We've seen too many alien movies today. Radio audiences are too desensitized for something like War of the Worlds to play the same way now as it did then. Ask yourself what the current trends are, what the public is concerned about and how you can turn those subjects into a mock radio broadcast people will believe.
Write in terms of what your audience can hear. Radio is all about bringing something to life with sound. Think about sound effects that might become a part of your script, the type of announcer that might read it and the specific words you use. A lot of this will be determined in the production phase of the script if it ever airs, but the more you can work into your script, the more realistic it will be. This is especially true of the words you write for the announcer.
Write your radio script in the proper format. A radio script is divided into two columns. The left column is where you indicate sound effects, abbreviated SFX. In the right column, in all capitals, describe the sound listeners will hear. The left column is also where you write character name or announcer. Announcer is abbreviated ANNCR. Write what the character or announcer says in the right-hand column. Abbreviations should be written in uppercase and speaking parts in lowercase.