For one to successfully complete this game: All columns and rows lines must be filled with numbers: 1 to 9 without repeating any number and each 3*3 box of numbers must have numbers 1 to 9 without any repetition.
It is advisable to start with the box, column or row with fewest missing numbers. Start with elimination method and identify the suitable number to fill in a particular box.
Guess work should not be used while playing this game. In the event that you get stuck in one box, don't dwell there; look for other boxes to fill and you'll eventually find the appropriate number to insert in the cell.
Remember Practice makes perfect. The more you try the better you become.