First step
Realize your not the front liner, far to often assassin players in low elo assume all because they are melle fighters that it means they are front liners. Being mellee does not = front lining
If you are a yi, a yalon, a zed, or a yasuo, your job is 1 of 2 things.
1-Find the squishiest target, kill them the moment they miss step and get out
2-Have your front liner start a fight, kite in and out of the fight, to then be the final form of clean up after most the people on your team and the enemy team is dead
Want to know why a Katarina or master yi is normmally the last person alive in a huge team fight? Because they waited for the right moment to go all in and kill everyone, your job is not to start fights, your job is to clean them up. To find out the proper in order in who needs to die first and execute it accordingly. People think playing assasins takes no skill and that it is stupidly easy, but these same people are also stuck in bronze/silver, playing a assassin of any kind requires understanding the game state, the fight state and how to execute the proper kill sequence and when to do so. Typically u go ham the moment ur tank ccs the team or begins isolating somebody.