Open Vivaldi and Opera 15+
- go to https://addons.opera.com/en/extensions/details/smart-rss/?display=en
- click “Add to Opera”
- on the popup thingy click on “get it anyway”
- save to a place where you can find it again.
- open vivaldi://plugins
- drag and drop the extension file from your file manager to the vivaldi://plugins tab
In Opera 15+:
- right click on the extension’s button
- on the popup click “Options”
- in the options tab click on one of the “Export” buttons. I personally prefer “smart” because I want the contents of the feeds too, otherwise OPML, which works for some other readers too
- click on the new link that shows after you pressed the export button
- save it in a location where you can find it again
You can close Opera now.
- open the smart RSS tab by clicking on the extension’s icon
- click on the wrench icon
- in the popup click on “Options”
- in the options tab click on appropriate the import button (depending to which format you exported)
- select previously exported file
- wait until the import is finished successfully (there is some status information showing next to the button)
After those steps all feeds should be imported to Vivaldi.