Hold your sheet of paper so the shorter edge is on the top.
Fold the paper in half by taking the top edge and folding it to the bottom edge. Some people call this folding the paper hamburger style.
Fold the paper hamburger style again by taking the short edge (now on the side) and folding it to the other short edge.
Unfold the paper all the way. There should be two creases from your folding that divide the paper into quarters.
Hold the paper so the shorter end is at the top. Cut off the top left quarter of the paper by cutting along the crease lines created by folding. Cut on the creases so one quarter of the paper is completely removed from the sheet. This will leave you with half a sheet of paper on the bottom with a tab on the top right. Discard the quarter of paper that was cut off.
Arrange the paper so the tab is at the top. Fold the tab down at the original crease, over the half sheet of paper. This creates the inside pages of the book. Then, fold the remaining side at the crease over the tab. This creates the cover of the book.