In general, it is best to put in awake but drowsy. That helps baby learn to self soothe and fall asleep without being rocked in your arms or with a breast in the mouth. (Rocking in SNOO is not a problem, because it will be gradually reduced later to wean your baby off of motion.)
However, if your baby is having trouble getting accustomed to being in SNOO and fusses more when put down, it is best to first fully calm your baby (swaddle + turn on a strong white noise + feed and then rock or lull to sleep) and gently transfer your sleeping baby into SNOO.
Note: Using an extra white noise in the room when you do the transfer helps babies not wake up when you turn on the bed sound and motion.