I'd read that there was no point starting a routine until 6 months but all anecdotal evidence convinced us otherwise.
We were really struggling with DD and getting her down for naps in the day. She would be so obviously tired but would not drop off, leading to a hysterical baby and us at our wits ends, walking round the block just to calm her.
We started with a solid night routine that at first took hours to implement but now is down to 30 mins. Bath, massage, bottle, into sleeping bag, story, into cot. We give her a dummy and a comforter and turn on white noise. Then say night night DD, time for sleep. We 'sssssh' until she is relaxed and then leave the room. More often than not she'll go to sleep.
Daytime naps don't go so smoothly and often need more sssshing. At first she would scream when we took her near the cot and we realised she was overtired so now we preempt that. We try not to feed to sleep and do feeds on waking instead. Same routine with into sleeping bag, story, cot, comforted and dummy. The longest she has cried is 50 mins... we don't leave the room because that's not something I could deal with. We let her know we are there and sssh her but that ultimately she must go to sleep.
Yes it may seem cruel to leave her to cry, but actually we thought it was crueller not to teach her to sleep. She would often be fraught all day and is so much happier now she is going down for naps.
We still have two night wakings (11pm and 4am) but she's only 4.5 months so I don't expect her to drop those anytime soon.
I agree that everyone needs to sleep. Babies and parents. You can meet their needs as well as doing sleep training in my opinion.