How to snowball?

I always fail to snowball. Like I will end up 6-0-4 by minute 15 and end the game losing with 11-7-8.

I know that after a few kills someone ain't worth that much anymore but my farm is also the highest of everyone everygame. Yet I seem to only get weaker later on in the game. Which is obvious since I play alot of early game champs but still I should stay strong considering how fed I get every game.