Tap once to fill the desired square. Tap once more to fill same square with cross to mark as empty square. Tap once more to clear the square. Or you can hold and fill more than one square at once.
In order to solve a puzzle, one needs to determine which cells are going to be boxes and which are going to be empty. Determining which cells are to be empty (called spaces) is as important as determining which are to be filled (called boxes). Later in the solving process, the spaces help to determine where a clue (continuing block of boxes and a number in the legend) may spread. Here we use a cross to mark cells that are spaces for sure.