Complex sentence: Southern, Fonda and Hopper met in New York City in November 1967 to develop their ideas and these brainstorming sessions formed the basis of the screenplay that Southern then wrote from December 1967 to April 1968.
Split into simpler sentences: Southern, Fonda, and Hopper met in New York City in November 1967 to develop their ideas. These brainstorming sessions formed the basis of the screenplay that Southern then wrote from December 1967 to April 1968.