Click on your profile picture in the top right corner:
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Press 'Plugins' in the dropdown menu:
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Follow the link to the Figma Community:
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Type in the name of the plugin in the search tab:
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Click on the 'Install' button:
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Right-click anywhere on the board or editor, hover over Plugins in the menu and select a plugin to run it:

You can log in to your Icons8 account right in the plugin by clicking on the three lines in the top right corner:

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