My LO is 13 weeks and we started a bedtime routine around the 8 week mark. She was not sleeping very well at night (she would be up for many hours at each waking), and we had a lot of trouble getting her to fall asleep.
I put on a lullaby CD and dim the lights while we diaper, nurse, change clothes, and read a couple of board books. We play a little bit, but I keep my voice really quiet and slow movements. I think this signals to her that it's getting to be night time. Then I usually swaddle her, nurse her a little more, do a little rocking, and put her in her crib while she is still a little bit awake. I turn the lights totally off and put some static on the radio. She is usually asleep within a few minutes of laying down in her crib, but I usually leave the room. The whole thing takes maybe 30-45 minutes.
I think she's sleeping fairly normally for her age. She gets up about every 3-4 hours to nurse, but goes back to sleep very easily. I am one tired mama at this point, and will admit that I'm jealous of my friends whose babies are sleeping 7-8 hours. However, I think she's just a kid that needs to eat frequently, even at night. We co-sleep sometimes, but I think she actually sleeps more soundly by herself. She seems to wake up more cheerfully and rested in the morning when she's been in her crib.
In any case, I'm glad I started this routine, because she's my first, and I think it's setting us up to have good bedtime habits for when she gets older (for us as much as for her).
I read the book No Cry Sleep Solution, which kind of helped. One thing is that I start the bedtime routine before she's obviously tired or cranky.