Lesson 4 Very important hints.
One of the biggest mistakes beginners make is to put all possible numbers in each cell. This a lot of work and not necessary until you get to the very advanced puzzles. If a possible number can go in a row, column or block more than twice do NOT put that number in those empty cells. If a possible number can ONLY go in a row, column or block twice then put that number in each of the 2 cells.
As you progress you will find that this is technique makes it not only less messy, but easier to solve.