1. Go home by clicking on the home icon (top left):
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2. Click on your account settings gear (top right) to see this menu:
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3. Click on "Import or upload sites" and see the button to pick an OPML file:
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4. Upload the file and wait a while. You'll probably need to reload the page. Importing hundreds of new feeds can take 5+ minutes sometimes.
Similar instructions for Feedly, The Old Reader, and Digg Reader.