The most important thing to remember when thinking of sling safety is the word T.I.C.K.S.
Tight - Slings and carriers should be tight enough to hug your baby close, as this will be most comfortable for you both. Any slack/loose fabric will allow your baby to slump down in the carrier, which can hinder their breathing and pull on your back.
In view at all times - You should always be able to see your baby’s face simply by glancing down. The fabric of a sling or carrier should not close around them so you have to open it to check on them. In a cradle position, your baby should face upwards not be turned in towards your body.
Close enough to kiss - Your baby’s head should be as close to your chin as is comfortable. By tipping your head forward, you should be able to kiss your baby on the head or forehead.
Keep chin off the chest - A baby should never be curled so their chin is forced onto their chest as this can restrict their breathing. Ensure there is always a space of at least a finger’s width under your baby’s chin.
Supported back - In an upright carrier, a baby should be held comfortably close to you so their back is supported in its natural position and their tummy and chest are against you. If a sling is too loose, they can slump, which can partially close their airway. (You can test this by placing a hand on your baby’s back and pressing gently – they should not uncurl or move closer to you.)