Start with full lines
If a row or column clue equals the grid size, fill the entire line.
If a line is fully solved, mark the rest of that line with X marks so you do not accidentally fill it later.
Use overlap on big clues
On a 5-wide line, a clue of 4 can start in only two places. The middle three cells are filled in both placements, so they are safe.
Switch direction often
Every filled or crossed cell changes what is possible in the perpendicular clues. That back-and-forth is the core nonogram rhythm.
Practice this in the app
Start with 5x5 lessons, then move into prettier 8x8 picture puzzles.