See it, fill it
1-step reasoning
At this level every conclusion takes a single observation: you look at the digits already placed and see that a cell can take only one digit, or that a digit has only one place left. No notes, no assumptions.
There are two questions to ask the board. From the cell: "which digits can go here?". From the digit: "where can this digit go in the unit?". Most easy and medium puzzles are solved with just the four techniques below.
Where does this digit go?
Pick a digit, cast rays from its copies, and find the only empty spot in a box, row or column.
Which digit is this cell?
Pick a cell and collect the digits in its row, column and box. If exactly one is missing, you are done.
When to use it: Always the first thing to look for, and the thing to return to after every elimination at the higher levels.
4 techniques
Full House
Last Digit
A row, column or box has only one empty cell left: fill in the missing digit.
Difficulty
Hidden Single in a Box
Cross-hatching
Pick a digit and a box, cast rays from the copies of that digit, and find the only cell still able to take it.
Difficulty
Hidden Single in a Row or Column
Hidden Single (line)
In a row or column, a digit has exactly one cell left where it can go.
Difficulty
Naked Single
Sole Candidate
A cell sees 8 different digits in its row, column and box, so only one possibility is left.
Difficulty