This technique is very easy – especially if you’re using pencilmarks to store what candidates are still possible within each cell.
If you’ve managed to rule out all other possibilities for a particular cell (by examining the surrounding column, row and box), so there’s only one number left that could possibly fit there – you can fill in that number.
Here's an example:
In fact, filling in all of the possible pencilmarks (as many programs will do for you), you'll see the following grid, and you can easily spot several single candidates:
Once you’ve filled these in, you’ll soon see plenty more single candidates, and with this technique you may be able to go on to complete many of the simpler Sudoku puzzles!
If you’re using a computer program to assist you, then you’ll probably do most of your placements with this method.
If you’re doing your pencilmarks by hand – double check that you’ve filled them in otherwise you might make a placement that isn’t valid!