Open the Facebook application in your mobile device.
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Click the 'Menu' section on the lower right side of the screen.
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Click the Facebook page you want to connect your Instagram account to.
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Once you've switched into your Facebook page, go to Settings & Privacy > Settings.
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Scroll down until you reach the "Permissions" grouping then from there, click "Linked Accounts", then click "Instagram".
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Click "Connect account".

You'll be prompted to log into your Instagram account and asked to once again 'Confirm'.