The rules take one sentence: fill the grid so every row, every column and every three by three box holds each of the nine values exactly once. That is all of it. Everything below is about how you work out where each one goes, because you never have to guess. A properly built sudoku has exactly one answer and a path of pure reasoning that reaches it.
The five techniques here are in the order most people learn them, and they are the same five the difficulties in this game are graded by. A puzzle is called Tricky because it genuinely needs hidden pairs, not because it starts with fewer tiles. Clue count is a poor guide: a grid with fewer starting tiles is often the easier one.
1. The last free square
The first thing to look for, and the whole of the Cozy difficulty. If a row, a column or a box has eight of its nine squares filled, the ninth can only be the value that is missing. No thought required, just the observation.
Eight are placed, so the ninth square has to be a 9.
It is worth chasing these repeatedly. Every value you write in can complete another row or box, so one placement often unlocks three or four more in a small cascade. Solvers call these naked singles.
2. Scanning
The technique that carries Gentle. Instead of asking what goes in a square, ask where a value can go. Pick a box, pick a value, and rule out every square in that box that already sees a copy of it in a shared row or column. If only one square survives, you have found it, even though that square might still look like it could hold several things.
Those four 5s rule out every square of the top left box but one, so the 5 goes in the highlighted corner.
This is called a hidden single, and it is the workhorse of ordinary solving. Most people scan by value: take the 1s, look at all nine boxes, then take the 2s, and so on.
3. Locked candidates
Where Steady lives, and the first technique that removes possibilities rather than placing a value. Sometimes a value can still go in two or three squares of a box, but all of them sit in the same row. You do not know which square it is, and you do not need to. You know the value is somewhere in that row, and therefore it is not anywhere else in that row outside the box.
The 7 in this box has to be one of the two marked squares. Both are in the third row, so no other square in that row can be a 7.
The same argument runs the other way, from a row into a box. If every place a value can go within a row falls inside one box, it is ruled out of the rest of that box. Those two together are usually called pointing and claiming.
4. Hidden pairs and triples
Tricky asks for this one. Once you are keeping pencil marks, look for two values that between them can only go in the same two squares of a unit. Those two squares must hold those two values in some order, so every other pencil mark in both squares can be crossed off, even if there were four or five of them.
It has a mirror image. If two squares in a unit have only the same two candidates left, that pair is locked into them and can be cleared from every other square in the unit. That version is a naked pair, and it is easier to spot. Both extend to three values across three squares, and rather less often to four.
The habit that makes these findable is tidy pencil marks. This game will keep them for you: turn on Show candidates in settings and it fills in what each empty square could still be, or turn on Tidy notes and it rubs out the marks a tile you place has just ruled out.
5. Wings and fish
The Devious tier, and the point where a sudoku stops being a scanning exercise. These techniques chain a claim across two units at once.
The X-wing
Find a value that, in two different rows, can only go in the same two columns. Whichever way round it falls, those two columns each end up with the value inside one of those two rows. So it is ruled out everywhere else in both columns.
The only places a 3 can go in those two rows form a rectangle, which clears the 3 from the rest of both columns.
The XY-wing
Three squares, each down to exactly two candidates: a pivot holding X and Y, and two squares it can see holding X and Z, and Y and Z. Whichever value the pivot takes, one of the other two ends up as Z, so any square that sees both of them cannot be Z.
The order you should learn them
Work down the list, not up. Almost every position yields to something earlier than you fear, and reaching for an X-wing on a grid that still has a plain hidden single sitting in it is the most common way to make a hard puzzle feel impossible. When you are stuck, scan again first.
If you would rather practise a technique than hunt for it, the difficulties here are built for exactly that. Each one is graded by the hardest technique it actually requires, so playing Steady repeatedly is a locked-candidates drill and nothing harder will show up.