Square faces have roughly equal lengths and widths. The forehead, cheekbones and jawline are almost the same widths. The chin is noticeably flat or square looking.
If you have a square face, you have a strong jawline, and you may wish to draw attention away from it.
How to achieve it
You should use your eyebrows to balance your face by introducing another strong visual feature. The way to do this is to introduce curves to your eyebrow shape as a balancing visual feature. The stronger your jaw, the more curve you can get away with. The more defined and sharper the peak of your eyebrows, the stronger the impression will be. You could achieve this with angled eyebrows, but you don’t want a very high or sharp angle
Quite thick and well-defined eyebrows can balance a strong jaw. Thinner eyebrows can look out of place in a square face which has bold features.