You contact the copyright holder and ask them if you can use it. If they say "yes" send them a release form. You can send a "blanket" release that indicates you have "perpetual, universal" use of the material as it pertains to your project; that will probably co$t you. Then there are festival release forms where you agree that the copyrighted material will be used only for festival showings. If you happen to sell your project or get distribution you will have to renegotiate.