If you have video proof, start a support ticket in general questions and state it's about reporting a user. Add as much information as you can such as a link to the match history, the persons summoner name of who you're reporting, screenshots/video evidence and anything you may think is something they need to know. The person who checks it will move it on to the team that deals with the bans and such.
Keep in mind that, like TheCurator said, the Red who responds will be unable to tell you if the person gets banned or not. But if your proof is real and shows that the person was in fact feeding on purpose he will be dealt with.