[Art] How do I position the nipples? NSFW

Deftly, skillfully, between your teeth.

Joke's aside, I've found success with envisioning the globular weight of a breast gathered in a pseudo-orb (different breasts are different; adjust as needed) to inform where on that centered vertical line its nipple should go. Begin from the point on said frontal arc where the nipple aims laterally (perpendicular to spine) when the body is standing upright.

Then, adjust to taste: Lower for pointier breasts, higher for perky, as well as wider areolas for puffy or slimmer for button nipples, et al.

Use references?

Whether from others' art or from actual photographs, if you find one with a similar body type in a similar pose, then that can give you guidance ... at least until you develop visual memory for the 'more interesting' poses as well. And there are so many existing images online you could reference -- there's got to be some out there that are similar enough to work from.

Use references. But there's quite a bit of variety.