Take your time and do not rush. KSudoku is not a quick game.
Pay attention to the row, column, and block colorings. They are there to help you.
Make sure that no duplicate symbol exists already in the row, column or block where you are entering a symbol.
If in doubt, try to foresee if a square you are entering a symbol into may be needed by some other symbol, which otherwise cannot go anywhere else due to the row, column and block rules.
Use the Internet to search for general hints and strategies for solving Sudoku puzzles.
Use marker moves when you spot a square or squares that have just a few alternatives but you cannot decide between them immediately. Markers will save you time and memory effort later, when more squares have been filled in. See How to Play for details of how to use markers.
If you are having trouble, try to retrace your steps and see exactly where you have made an error. You can use the → and → menu items, repeatedly if necessary, and there is a setting in → that highlights errors.
Even though KSudoku provides a 'Hint' option for you, try not to use it often. Remember, solving the puzzle all by yourself enhances your brain activity, while using the 'Hint' feature too often is cheating.
If you have too much trouble solving a certain puzzle and simply must use the 'Hint' feature, then maybe you should try decreasing the difficulty level, or try a simpler, more basic puzzle.
Some puzzle layouts feature very specific game board shapes. It is a good idea to have the highlighting option enabled even if you consider yourself an experienced player.