1- Search for the row or column that is the most filled and try to find a spot
easy to fill
2- Search also for the big square that is mostly filled and try again to find
easy to fill spots.
3- Try to focus one a specific row or column in one of the big squares. If you
find out that the 2 neighboring rows and/or columns cannot contain a certain
number, then try to place it in that specific row / column.
4- Do not fill a number if you don't have a very high chance for it being
there, because that means you may work on the wrong path for a long time then
discover that that choice was wrong
5- After each row, column or square fill, make sure you are not affecting
other rows, column or squares in a way that makes it impossible to continue.