This short guide is just the tip of the iceberg when it comes to Sudoku. The strategies outlined in the guide should be enough to complete any of the puzzles that are published in UK national newspapers, but it is possible to make harder puzzles.
The program used to produce the screenshots for this site is a simple program. When I wrote this program I had already used a far superior product by a chap named Angus Johnson. That product is called Simple Sudoku and has all of the features you find in the download on this site and a lot more besides.
The Simple Sudoku program is free to use and has an extensive solving guide in the online help. The Simple Sudoku program will also give you solving hints when you get stuck - this helps you to learn how to spot when to use each of the strategies outlined in the solving guide.
The progam seems to work fine for me in Windows 7 & 10, despite not being designed with these in mind. The online help in the program uses an old version of Windows help - you may need to download a program from the Microsoft site in order to view the help file.