The best trolling you can do in groups. Just one buddy is sufficient.
Act like you don't know each other and have conversations that make everyone go
"wtf are these guys retarded!?".
You can argue with your buddy in team chat e.g.
The lower the elo that you play in, the funnier reactions you get. If you are at silver III or below you can even do "troll builds" to get your enemies upset cause they won't know how to deal with something they've never fought before. The key here is to play something untraditional. Not just something unconventional like AP Trist.
I and a buddy once just danced for a whole 20 minute at our tower, even after it was destroyed we kept dancing until the 20 minute mark. The early game was over and teamfighting started so we just steamrolled the enemy team there and gg. Needless to say our team raged at us nevertheless for not doing anything 20 mins long lol.
You can also do some really irritating things to your enemy like playing really horribly at unimportant moments. And once you get to a moment that actually matters like a dragon fight or team fight, you go full efficiency-mode. Your enemies won't see you as a threat and either ignore you or think you're an easy kill making them prone to kiting and overextending.
Also spasm with ctrl+1,2,3,4 while moving, autoattacking and other animations can irritate your enemy really hard.
The more people you got to troll with, the more awesome troll stuff you can pull off. E.g. if you got 4 people in total there'll only be 1 guy on your team who is solo queuing. Our team followed this single guy wherever he went to the entire game with champs that could all keep enemies off from us like Nidalee, teemo, twitch. The enemy couldn't do much, we just pushed all lanes together for the win.
Or if you got 3 people in total, that's enough to make a heavy CC lvl1 team. Get something like Heca, Ali and Xin(something with CC+ damage at lvl1 and then wait together in a lane bush at top or invade like crazy. After that, split to farm and once every while you can gather up for insane 3-man-ganks that nobody can get away from.