Question regarding paint-by-numbers for adults

[LunaSea]

2. No "correct" approach.

You can fill the spaces any way you like as long as the right color goes on the right number.
Unlike traditional oil approach (background to foreground) or planning underpainting, you can apply them in any order you want. Top to bottom will keep your sleeves and elbows free(er) of paint. A Mahl stick might help if it's a big image. Perhaps apply a mural approach, mentally grid it into quarters, eighths, or more and complete it by section. It's really up to you.

Lunatica is right about the lack of creativity involved, unless you desire to muck around with the colors, and you are free to do it any way you wish. But the exercise could bolster you confidence as well as tuning up your fine motor skills.