The best way to do this is to contact his publisher, Kobalt Music Publishing, and inquire about obtaining a sync license. A sync license is a legal agreement that allows you to use copyrighted music in your film in exchange for a fee.
The cost of a sync license can vary depending on a number of factors, such as the length of the song, how it is used in the film, and the size of the audience. However, you should expect to pay several hundred dollars for a single song use.
In addition to obtaining a sync license, you may also need to pay royalties to Elliott Smith's estate each time the film is exhibited. The amount of royalties that you will owe will depend on the terms of the sync license.