Tired, full, and comfortable: keys to a sleeping baby

There is no magic solution to make a baby sleep, but here are some suggestions that will get most babies to rest. Babies need three thing: to be tired, to be satiated, and to be comfortable.
The first two are easy. You can’t put a baby to bed if they aren’t tired, nor will they be easy to get to sleep if they are overly exhausted. So as soon as they are showing signs of fatigue you should be beginning the bedtime routine. Signs of tiredness are crying, rubbing their eyes, and general crankiness. For a baby a bedtime routine can be as simple as putting on their jammies and singing a song.
Now that you have a tired baby who is ready for bed you have to ensure that they have a full belly. For babies under three months getting their belly full can be a long process. This is why many babies in the first few months have a witching period in the evening. This is usually a period of 2 to 3 hours where the baby is on and off the breast and seems insatiable. What the baby is trying to do is fill their belly so that they can have a longer sleep. It can be frustrating for a parent but just remind yourself each night that this has a purpose and will end. If you are nursing do not be averse to handing the baby off to someone else for a short spell if you are finding it exhausting and to give your breasts a chance to fill a bit.
The last part is the most difficult because comfort can mean many things to a baby. It can mean a clean diaper. It can mean clothes that are comfortable. It can mean being the right temperature. It can mean wanting to be swaddled or wanting their arms free. It can mean sleeping on their back, or some babies are only willing to sleep on their stomachs. You will have to determine the conditions your baby likes best. One of mine liked to be swaddled on their back with some white noise, while the other hates their arms trapped, sleeps on their tummy and in a quiet room.
I won’t go into too much detail on how to make baby comfortable and feel secure. There are many articles online that cover babies needs and wants. That being said, I don’t buy into the theory that we should virtually be re-creating the womb setting. Babies are very adaptable and every moment of their day is a reminder that they are not in the womb anymore. After a few nights baby will be very aware that the setting is different and begin to adapt. What you want to make them feel is that they are near a mother or caregiver who makes them feel safe. One simple thing I recommend is to have the mother wear the crib sheet under her clothing for part of a day so it smells like her for the baby.
Her is a sample of my routine: at the same time each evening I get my baby changed into their pajamas and take them up to their dark room. I sing a lullaby as I nurse. Some evenings I include a bottle of pumped milk to my nursing. I nurse from one breast and then burp the baby. I then either give a bottle or the other breast and burp the baby. I keep repeating nursing and burping until the baby is very floppy and tired. When I reach the point that I feel like they put into their crib, I do so. The point of this process is to make sure the babies is full, comfortable (meaning they don’t have any gas bubbles), and tired. By continually moving them around between breasts and burping they become used to falling soundly asleep throughout movements and makes it easier to transition them to the crib at the end.
Beyond the baby’s needs, as a parent, we must be patient. The most difficult nights I’ve had getting my babies to sleep are the nights where I have been impatient and unwilling to take the time to go through all the steps. If you get frustrated, you skip steps and try to rush them to sleep or rush to get out of the room. If your child isn’t satisfied you are just sabotaging all your efforts by hurrying them.