Paper corners
Take 12 sheets of paper and cut them in half so that you end up with 24 strips measuring 15 x 7.5 cm.
Then attach double-sided tape to the short end of each strip, remove the protective film and stick the strips together to form a roll. Then press one end of the roll flat. Ideally, the adhesive edge should be neither exactly on the edge nor exactly in the middle.
Then glue the flat end together with a little craft glue. Be careful not to glue directly to the edge if you want to have patterned edges later.
You can also use double-sided tape for this step. But it's easier and faster with the craft glue.