Fold the paper until a dot shows through every hole. A fold turns its side over onto the rest, so the hole has to land on the dot. A dot that has been turned over only shows through the paper, and does not count until it is turned back. Either side can fold if it fits on the other.
Drag a square across a fold line. The paper follows your finger; let go past the line and it lands.
Or tap a square, then a highlighted fold line.
An arrow at the edge folds along its line. Where either side could fold, it asks which.
To unfold, swipe the folded side back the way it came, or press the arrow on its edge. Undo does the same.
Hint previews a fold. Press it again to apply. Where no fold can finish, it points at Undo.
More controls
Tab to an edge handle. Enter folds the smaller side; Shift+Enter folds the other side if it fits. On an equal crease, Enter folds left or top.
Arrow keys move between cells. Enter or Space picks a cell. Z undoes a fold. Escape cancels a picked cell or closes a sheet.
Restart clears the sheet; Undo restores it. The fewest possible folds appear after a solve.