The Baby Carrier Industry Association has a nice page up with basic safety tips for using baby slings, especially the extra attention that should be paid when carrying a preemie, newborn, or a baby who is ill or has other special needs.
The main thing to know is that – sling or no sling – an infant is safest when a caring adult is close by and keeping them in sight. Good sling positions good baby close enough to kiss, and with their face visible – that is, don’t let the sling or your body or a blanket or anything else obstruct the view of your infant’s face, especially of course their nose and mouth.
Other common-sense reminders include reading the product instructions carefully, and keeping the sling snug enough that baby’s position does not shift around after you put them in. If they start off in a good spot, “visible and kissable” but slide downward when you move around, that is not good! Read more from the official baby carrier industry group at this link
You also might be interested in the Consumer Product Safety Commission (CPSC) in their campaign to educate parents on safe baby carriers (link) and their work with the BCIA to spread good information. See this press release about the BCIA’s agreement with the CPSC on this need, and see how BCIA works with Health Canada to this same shared goal.