Start by scanning
Choose a digit and look at where it already appears. Each 1 rules out another 1 in its row, column, and box. If a box has just one cell left that can take a 1, fill it in. Then scan for the next digit.
“Pick one number and follow it across the board.”
Altair
Naked and hidden singles
A naked single is a cell with just one possible digit. Check its row, column, and box to rule out the others.
A hidden single is a digit with just one possible cell in a row, column, or box. That cell can still look like it has other options when you check it on its own.
“Start with the cell or start with the number. Either way, look for one possibility left.”
Juno
Use notes for pairs
When scanning stalls, use Notes to write down every digit that could fit. If two cells in the same row have exactly the same two possibilities, those digits belong in those two cells. Remove both digits from the notes in the rest of the row. This is a naked pair, and the same idea works in a column or box.
“You can clear the other notes before you know the order of the pair.”
Orion
Moving up to full Sudoku
Full Sudoku uses 1 to 9 and has 3 × 3 boxes. The logic carries over. Start with a scan, look for singles, then use Notes to find pairs. After a placement, scan again to see what it opens up.