A good time to begin a "going-to-sleep" routineis when your baby is "settled in" -- about three months old.
Into the Crib Awake
Many young infants are accustomed to falling asleep at the breast or while being rocked in a parent's arms. This works well for the first two to three months. But between three and four months, it is very important to start putting your baby down in the crib awake every time she goes to sleep. She needs to learn to fall asleep on her own. Do not put your baby to sleep with a bottle. It can cause tooth decay and dependence on feeding as a way of going to sleep.
Go through your bedtime routine (see below), then swiftly and smoothly put the baby in the crib and say your "good night." If your baby really fusses, stay with her until she goes to sleep but try to use just your voice to calm her down. If she gets really mad, you can try to pat her back, and if that does not help, pick her up, soothe her for a couple of minutes and put her back down in the crib, still awake. Don't let her fall asleep in your arms! This can be very trying with some babies. You may have to repeat the "picking up-soothing-putting down" routine over and over, for up to an hour on the first night. It'll be much easier on subsequent nights.