Every empty cell needs a number. Add or multiply the numbers across a row — using the symbol in the corner — and they should hit the target to the right. Do the same down each column to hit the target below it.
The colored pill under the grid tells you what kind of numbers are allowed. It changes color if something you've entered doesn't fit.
Tap a cell to open the number pad. As you fill things in, some cells will fill themselves in — once only one cell in a row or column is left empty, its value is already decided by the target.
Cells shown in pink are givens: they are correct and can't be changed. Some puzzles start with a few. If you're stuck on a 3×3 or 4×4 puzzle, press Get a Given to have one more cell filled in for you. It never gives away the whole puzzle.
Press See All Solutions any time to see every grid that works. Nothing you type is ever wrong until the whole grid is complete.
Strategy
Start with whichever row or column has the smallest target — there are fewer ways to make a small number, so it narrows things down fastest. Once a row or column is one cell away from full, that last cell is never really a guess.