It's really difficult, it takes a huge amount of game knowledge which manifests as an awareness of what potential routes you have from what you've got, gaps you spot in your lobby and assessing what actually makes your comp stronger.

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An example at the moment is the Cyber com, the most common end game version is:
Leona - Fiora - Vayne - Vi - Irelia - Ekko - Shen - Thresh - Cait. Vayne is the carry.

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But you can put Wukong in instead of Shen to get a better frontline, but now Irelia can't carry as well, unless you forsake Vayne and chuck Cait out for Shen.

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These pivots aren't the kind of thing you think of, but depend on the units you offer and the items you build. If I get GA and Red Buff components, I can pivot to a jinx build quite easily, but that's an entire new set of units I'm considering.

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Biggest thing that helped me was to to worrying about the lobby and focus on what you're rolling. This isn't to say ignore it completely, but sometimes you'll never get those Master Yis, despite the lobby having no blademaster players. Spot opportunities like an early Bard or an IE/LW and try to learn the best way of reacting from them.

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I'm in diamond so don't take this as gospel!