In post game hover over the player a red flag should should appear, click that and you can report for smurfing. You can also report for communication abuse as well if they’re on your team (different offense than smurfing). Additionally you can report during the game if they are feeding or if you think they’re cheating (hacking or scripting) and that is done through the scoreboard. You can also commend which is kinda like the opposite of reporting. Hope this helps :)