From my experience ,these are some aspects that help my accuracy:
Slinging relaxed,but sharp. 70-80% power
Focusing on the middle of the target. Aim small,miss small
Being aware of the release knot, and how it pulls on my hand,i.e. feeling the sling, sensing, where it is while rotating.
Clear mind. not thinking too much. or at all ,at least when throwing.
Consistent ammo,consistent throwing moves.
Stepping into the throw, just in the right time, can really increase your time window to release,for an accurate shot.
Being aware, that the sling releases way away from your body. The closer you sling at, the sharper this angle of your vision, and the angle of the release is.
You got to get the feel of it,and let it happen, by not trying to do it,letting it happen, by being aware of every aspect of it, not overthink it,just sensing, how does it work. My most accurate shots ,always come with a riding the flow feeling. While rotating, I got the feeling that,I'm on,and it's 100% HIT, I pull, and BAMM it's dead on. I just wish, I'd have those much more