The function can be reached by right-clicking a player’s name at the post-game screen. From there you can report for verbal harassment, intentional game disruption, excessive idle, or “other.” You can also leave comments on the situation. I’ll be curious to see what Riot actually does with the reports. Not much has been said yet, so I doubt they’re getting the deluge of complaints that will doubtlessly come as more people discover the feature.

In any case, I’m glad for the option, but I’ll wait to pass judgement until we see how it will impact the game.