Here are some clear instructions on how to lose a won game(or any game at all):
- Go solo as much as you can.
- Chase kills. (this one is really important)
- Don't protect your carries.
- Don't listen to anyone, just do what you want.
- Blame others as much as you can.
- Don't play safe, just go for the kill and feed.
Now, if you don't want to lose, do the opposite. It's not that hard!
your average lol player