Step 1 : Check the stroller has a seat that suitable from birth.
The seat should convert to almost or completely lie-flat when you unclip or unzip the fabrics. A lie-flat position makes stroller suitable right from birth, as the flat position is healthier for the growing spines of newborns
Step 2: Make sure the brake is fully locked
Make sure the brake is fully locked and the seat is fully reclined. Some strollers differ but converting usually involves adding the newborn nest or liner
Step 3: The crotch strap
Place your baby gently in the seat. Legs go either side of the crotch strap. Bring the shoulder straps over the arms. carefully fasten the harness Then adjust the straps so that your baby is fully secured
Consider a carrycot if your newborn will spend long period being pushed around. A carrycot offers more protection from the outside world, and a firm, flat base is better for your baby's growing spine and breathing
Extract from: https://www.youtube.com/watch?v=-SeVdy1f1uc