Put characters from A (or 1) up to a given maximum in some of the squares, so each character appears exactly once in each row and column.
The rules vary depending on the game mode, and the Type menu has a section for each:
On a touch screen, a tap does what a click does, and a long press what a right-click does (more ways to right-click). A key the puzzle’s controls show as a button can be pressed there instead of on a keyboard.
Left-click to select a cell, then type a letter or number on your keyboard to enter it. Press Backspace or Space to clear a cell.
Press ‘X’ to mark a cell as empty, or press ‘O’ to mark a cell as “definitely not empty”; ‘-’ and ‘_’ do what ‘X’ does, and ‘+’ and ‘=’ what ‘O’ does. 0 clears a cell as well. A letter and its number are the same key to the game, whichever the board shows: A is 1, B is 2, and so on.
Right-click a cell, then type a letter or number to add a pencil mark. Pencil marks can be used for any purpose. The letter ‘X’ can also be used to indicate a cell that might be empty. Pencil mode is sticky: once a right-click switches it on, it stays on wherever you click, until you right-click again. The Marks key is lit while it is on, and pressing the key also switches it. To have it last only until your next click, turn off Right-click toggles a sticky pencil mode (stays on until right-clicked again) in the preferences.
You can also use the arrow keys to move the selected cell around. Press Enter to toggle between entering letters/numbers and entering pencil marks.
Press Fill marks among the puzzle’s controls to write in every pencil mark wherever nothing is placed and no mark is written yet. The M key does the same, on any board where M is not something you type. Marks you have narrowed are left alone, and pressing again never starts over: see Filling in all the pencil marks. Once every empty cell has marks, the press crosses out of each what is already placed in its row and column.
Letters is the puzzle called ABC End View, whose designer is unknown. Numbers is the puzzle called Number Ball, invented by Inaba Naoki; the original puzzles are available here: http://www.janko.at/Raetsel/Nanbaboru/index.htm
Hint explains the next step rather than simply making it. When a step needs pencil marks, it starts by filling in every candidate, ‘X’ included, in each empty square that has none yet (the same as pressing Fill marks), then works from your own marks, so it carries on from wherever you are. If a letter, ‘X’ or ‘O’ you’ve entered is wrong, or a square’s pencil marks have crossed out its answer, it asks you to fix the highlighted mistakes first.
The hint speaks of squares, and a clue sees the first letter along its line. The pencil-mark ‘X’, the note that a square might be empty, is its empty-square mark, and a sentence that lists what a square can still be writes it as X. It is crossed out like any other pencil mark: a square left with only that mark must be empty, and a square that has lost it must hold a letter. Each row and column has a fixed number of empty squares, so counting them is half the game: once a line has all its empty squares, the rest must hold letters, and once it has all its letters, the rest must be empty. In Numbers, where every symbol is a number, the hint says “number” wherever this says “letter”.
On the hardest boards that need no guessing, a step may be a wing. Take a square whose pencil marks are down to two values, say a or b, and two squares it can’t repeat a value with, both outlined: one marked a and c, the other b and c. Whatever goes in the middle square, one of the outlined squares must have c, so a square that can’t repeat a value with either of them can’t be c. If the middle square could be c as well, the same holds for a square that can’t repeat a value with the middle square either.
These parameters are available from the ‘Custom type…’ option on the ‘Type’ menu.