The rule of 45
Every row, column and box adds up to 45. Use it to find single cells.
The numbers 1 to 9 add up to 45, so every row, column and 3×3 box totals 45. If the cages inside a box cover all but one cell, subtract their totals from 45: the remainder is the missing cell.
It also works across several rows: two whole rows total 90, three total 135. Add up the cages that sit entirely inside, and look at the cells that stick out.
This is the single most useful Killer Sudoku technique once the easy cages are done.
Practise it
The Hint button on every puzzle explains the next step in the same plain language, so you can learn as you play. Play a Killer Sudoku puzzle.