The ink on the sheet is wet. Tap a crease line to preview a fold, then tap it again (or press Fold). Ink prints onto the mirror square on the other side of the crease, in both directions.
Where two inks land on the same square, they mix:
Match the target in as few folds as you can. Par is the fewest folds possible. Ink never washes out, so if a square gets ink it shouldn't have, undo.
A new daily sheet comes out at midnight. On an unfolded iPhone Duo, the target sits on the left page and the board on the right.