Ideal Positioning Reference
As much as possible, the position your child is in when in a carrier or sling should mimic their natural position. This may change depending on your child's age and even if they are awake or asleep. It's very important to understand that awareness of the position of your child in a sling or carrier is not just important when you are putting them in, but its something to be monitored regularly as you go about your day carrying your baby.
Younger children tend to naturally hold their bodies in a C shape with their back gently rounded. As they get older they tend to 'straighten out' but this process can take longer than you'd expect, with many children still having a natural curvature to their spine until around 18 months old.
Your child will probably feel most comfortable and secure with their weight being supported from below and the ability to have their arms close to their face if they so wish. A position that it is useful to aim for is one where the child's pelvis is tilted, their back allowed to naturally curve and their knees and elbows bent.