How to end the game ?

Silver 5 player here, last season Silver 3. If anyone from Plat+ reads this, i want your help. I've lost many games because we were in a HUGE lead but didnt finish it at a right time. We take baron, but my team keeps only groupping up at mid and winning-losing the fights by luck. I cannot split push top/bot because someone will rage at me in /all and all of the 5 enemies will come for me. I'd please bronze/silver/gold players to not talk sh*t here because i know the same amount of sh*t as you do. Thank you
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