"I will be making my directorial debut from a script I wrote," Kosar told Shock Til You Drop, disputing rumours about Brad Anderson helming the project.
The writer also confirmed that the film will not directly focus on the famed Manson murders or solely on the titular character of Charles Manson, but that it will be "a Family biopic".
"A movie about a surrogate family of wayward teenagers who, through extraordinary circumstances, came together and were transformed into the most notorious American family of the 20th century," he said.
Kosar confirmed that the movie will follow members of the Family as well as their eventual victims, including Sharon Tate, who was murdered while pregnant.
It was recently announced that True Blood star Ryan Kwanten will star as Manson.
The Family will reportedly begin shooting in 2011.