Drowsy But Awake
Once your baby is smiling in response to your loving face (usually around 6-8 weeks from their estimated due date), it's a good idea to start allowing them to practice falling asleep on their own. You can do this gradually and progress at your own comfort level, but the idea is to start placing them down for naps and bedtime while they’re still awake, allowing them to take themselves all the way to slumberville.
"Practice" is a key word here because some babies and parents will take to this easier than others, and that's OK. Honestly, there is no rush, but if sleep is a priority in your family, then this is the earliest time that I encourage you to begin practicing. The goal is to offer practice on a regular basis to help them master this new and increasingly age appropriate skill. If your sweet baby becomes very upset, it's 100% OK to go to them, re-soothe in whatever way works, dust your shoulder off, and plan to offer practice again at the next sleep period, or tomorrow, or next week!
If your baby is having a hard time with practice, it could be that they need more practice, but it's also worth considering that you are not trying to put them down for sleep in an overtired state. Aim to offer sleep within 45 minutes since the last time they woke, and bedtime and the first nap of the day will be the the best times to practice.
The reason for offering this practice is so they won't be confused when they wake up between sleep cycles, wondering why the last thing they remember is being snuggled up with a loved one, and now all of a sudden they’re alone and don't know how they got there. Instead they will learn that their special safe sleep space is exactly where they are supposed to be during sleep, so when they wake up between sleep cycles and their perfect little brains are still asking for more sleep, they will know that they already have everything they need to bring themselves back to sleep.
"Drowsy but awake" is the ideal state for a young baby to be in, so they can drift off to a peaceful sleep without a struggle. So go ahead and start off by soothing them to the point where they’re barely holding those eyelids up...and then (this part is important!!) gradually try to get them down before that super drowsy state so they become more independent with the falling asleep skill.
However, don’t wait for an older baby, over 4 months old, to be this drowsy before putting them down. Once your baby is spellbound by the busy and exciting world around them, your goal is to put them down “Fully awake, but calm”.
At 6-8 weeks of age (from EDD) your baby is making connections and it's helpful to start introducing the positive sleep connections that will serve them well in order to build that strong healthy sleep foundation from the ground up, and avoid the need for "sleep training" later on.