Step 9: Look Around a quadrant.
If you've looked across the columns, and across the rows and there are all duplicates, then look in the group. If there is one number in the group that only appears in one block, then it's your answer.
See... There is no guessing in Sudoku. One of these situations has to occur. You just need to find them.
IN this picture the 5 only appears in one block in the group. It goes there.
Write it big, then treat it like a pre-printed number.
See where we're going, here?