Sleep Routine

I’m so confused on how to start a night time routine. My son is 11 weeks old and he has a pattern throughout the day of eat, play, sleep, eat, play, sleep. However as the day goes on he gets harder and hard to put to sleep. The past two days he just does not want to sleep. I want to go about starting a need time routine however when I tried in the past it always got thrown of by him either being asleep or eating. So how do you go about starting a night time routine at 7:30 if he is already sleeping or eating.