Sudoku looks like a maths puzzle but uses no arithmetic at all. It is pure logic: every row, column and 3x3 box must contain the digits 1 to 9 exactly once. Most easy and medium puzzles can be finished with just two techniques.
1. Scanning (hidden singles)
Pick a digit, say 7. Look at every 7 already on the board. Each one rules out its row, its column and its box. Now look for a box where only one empty cell is left that 7 could go in. That cell must be 7.
Do this for each digit from 1 to 9, then repeat. Scanning alone solves many easy puzzles.
2. Counting (naked singles)
Pick one empty cell. List the digits already present in its row, column and box. If eight different digits appear, the ninth is the answer for that cell.
Counting is slower than scanning, so use it when scanning stalls, starting with cells in crowded rows and boxes.
Pencil marks
When both techniques stall, write small candidate digits in the corners of cells. Patterns like two cells in a box that can only hold the same two digits ("naked pairs") then become visible.
Building the habit
Ten minutes a day is enough to improve steadily. Start with easy puzzles and move up a level only when you finish without pencil marks.
You can play daily puzzles at Sudoku on BlockBlast.co, and the site's games collection has Minesweeper and 2048 if you want something different.
Disclosure: written by the BlockBlast.co team.