This is the simplest strategy which is used at the start of a puzzle. The idea is to find any cell which can only be one value based on all the other cells on the board and the Sudoku rules.
These strategies do not require pencil marks to be entered, but at first you might find it helpful to keep track of cells you've already looked at.
Once you have completed all these steps, you start again from the top, until you find no solutions to any cell during a complete cycle. At this point you know that there are no more singles to be found and you move on to the next strategy.
Another strategy to find singles is to take one digit at a time and figure out where it could be on the entire puzzle board.
The Sudoku Tutor Lock Mode was designed to help with the Digit Search strategy. It allows you to Lock in a digit and then every cell you tap, will get that value. This dramatically speeds up the solving early on.
To further speed up the search for singles, you can use the Possible grid mode, which shows you where a particular selected digit can go, by highlighting these cells in green.
One more strategy that can be done without pencils marks, is to look at every cell and figure out if only one value can go in it, which would be the solution. This strategy is called Naked Single and is easier to do once the pencil marks have been put in.
If you tap on the following links on an iOS device which has the Sudoku Tutor app installed, it will launch the app and open the practice puzzle. Tap hint once the puzzle is open to see the strategy in action.Sample Puzzle 1 Sample Puzzle 2