I found a cheater, so how do I go about reporting them to Riot?

send in a riot support ticket and DETAIL what you seen or spotted the cheating player doing. if at all SCREEN SHOT/VIDEO CAPTURE the activity.

if you see the same player again, do not say anything about the report or fact you have reported them for possible infractions. just proceed as normal but make sure if you see them using a cheat or exploit, try your best to capture them in the act and send that in as a support ticket explaining the purpose of the report and include your other support ticket number/numbers.