The safety of your baby is of the utmost importance. You should always follow all safety guidelines that comes with your baby carrier. If learning to do back carriers in particular, always practice first with someone else or kneeling over cushion, and never do anything that doesn't feel completely safe. Your baby should always have their knees well up, so most of their weight is carried by their bottom. They should be held closely into you, well supported by the carrier, which should extend knee to knee, and be "close enough to kiss". If you carry your baby on your back, ensure that the back of the carrier is no lower than your child's armpits. Its fine for them to have their arms out of the carrier, but they need to be well supported below this point. Baby wearing groups are a great resource for learning how to use your carrier safely. You tube clips are also awesome for learning how to do new carries.
Baby Wearing Groups
Joining a baby wearing group will greatly increase your knowledge of baby wearing safety and comfort, helping you to get the most out of your baby carrier.
Slingbabies keeps a list of New Zealand groups.