What is the proper way to report an account for being boosted or bought??

So i was playing a normal game to try something new in my mind when i got that gold player on my team He was playing so bad and always typing in chat and crying i had to mute him i was wondering why a gold player would appear in my game of normals while i am silver and be soo bad he was almost like inting with play style and mentallity of low bronze so after the match i did a small reserch on him and it seems he has a boosted account or bought as he is sitting at a 40% win rate in all matches and 11% in ranked and the champions he had good win rate on (which are in ranked how he got gold) he doesn't play anymore which only made me more suspicious and i wanted to know how can i report such accounts and players after a game ??{{champion:101}} this was his champion in the match if that makes any diffrence.
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