I was working with triplets. And they had a 'specialist' in to help set a routine as 3 babies all on a different routine was too strenuous.
This person said that from 2 months, you can set this routine.
Wake baby at 6. feed and play and put to sleep at 7:30. baby must stay in the cot for 90 minutes (until 9). when putting baby down, say dudu or sleep or something, but always the same word. place a teddy in the cot that baby can see. this teddy must always be in the cot so he/she can associate it with sleep. if baby wakes during the 90 minutes, do not pick up baby. put dummy in mouth, no eye contact and say dudu.
wake baby at 9. feed and play until 10:30. put baby to sleep and only wake at 12. feed and play until 13:30. put to sleep and wake at 15:00.
this is where it gets tricky as you dont want baby to now sleep close to bedtime, so let baby have small naps here and there. bath at 6pm. feed and cuddle and sleep by 7-7:30pm.
If baby wakes later in the mornings, say 7am, just incorporate these 90 minutes accordingly. awake with feed and play for 90 minutes, sleep for 90 minutes. it took a week for the triplets to get use to it.
they still get a little niggly occassionaly, but everything is so much easier. they do not like to stay awake for 90 minutes, but we try stretch it to at least 80 minutes. generally they do sleep for the full 90 minutes though.