Once you have found all the singles on the board, you must start considering candidates for cells in order to be able to solve the puzzle. Writing in the candidates will help you figure out what candidates can be removed from a cell until you have only one candidate left, which is the solution.
Snyder Notation is a strategy which places candidates into every box if the candidate for the digit only occurs once or twice. Sudoku Tutor provides the Fill in Snyder Notation feature to automate this process.
Snyder notation is very effective at finding naked singles and naked pairs, the latter being very helpful in elliminating candidates from the boxes they are in.
Once all the Snyder Notatin candidates have been filled in, look again for singles to see if you missed any. If you find one, it might allow you to then easily find others, if it happens to elliminate one of the Syder candidates, leaving only one in a box.
Also look for naked pair (2 cells with the same candidates in the same group) which would restrict the candidates in the other cells in the group, potentially uncovering a single.
Once the Syner Notation no longer helps find any more singles, it's time to put in all the pencil marks.
Sudoku Tutor makes this very easy, by providing the Fill in Pencil Marks feature.
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