The overlap method

Find the cells a block must cover wherever it sits.

Imagine the block pushed as far left as it can go, then as far right. Any cells covered in both positions must be shaded.

Do the same with several blocks: push them all left, then all right. Where the same block overlaps itself, shade.

Then use crosses: cells that no block can reach must be empty.

Practise it

The Hint button on every puzzle explains the next step in the same plain language, so you can learn as you play. Play a Nonograms puzzle.