In Safari, tap Share, then RectoVerso. Choose the kind of cards and how many, and tap Make up to 20 cards (the button shows the number you chose). The cards are written right there, one by one; discard the ones you don’t want and tap Add, which counts the cards you kept (Add 12 cards). Close the sheet early and RectoVerso finishes when you next open it, every card waiting for your check; with RectoVerso’s notifications on, one says the cards are ready, and tapping it opens the app.
The same works for text you select in any app, a text file, a Word or PowerPoint file, or a PDF (share it from Files). A link on its own isn’t enough: open it in Safari and share the page. RectoVerso reads the page that’s already open, so it downloads nothing.
With Shortcuts instead: in the Shortcuts app, make a shortcut that takes Safari web pages: Get Article using Safari Reader, then RectoVerso’s Make a Deck, with the article’s Body as its text.