so it cannot yet be said whether or not the sixth cell will contain a box.
add the clues together, plus 1 for each "space" in between. for example, if the clue is 6 2 3, you have 6 + 2 + 3 + 1 + 1 = 13. the first one is the space between 6 and 2, the second 1 is the space between 2 and 3.
subtract this number from the total available in the row (usually the width or height of the puzzle). for example, if the clue in step 1 is in a row 15 cells wide, the difference is 2 (6 2 3 is 13, 15 