Here's a real life view of what you should be looking at...
You want to stick the wrench into this opening.
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Keep it near the bottom. Your goal is to engage with the nut, in order to prevent the nut from being able to spin while you manually remove the backing plate by turning it counter clockwise.
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The backing plate comes off very easy... Here's a pic of the nut you need to hold in place while removing the backing plate.
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That's the best way I can explain it. I hope it helps.