You don't have to have a 'routine'- just do a gentle wind-down in the same manner each evening- it doesnt need to be at the same time either, just be prompted by her.
I'm assuming she's demand fed, so after feeding, maybe go to the bathroom together, wash her hands, or bathe her, or just have a little quiet massage time (not long, but long enough for the wee to work through her system, so 20 minutes) change her nappy, her sleepsuit if necessary, then bedroom, soft lighting, a 'story' or song, and snuggles, and then into bed for sleep, or sit with you snuggled until she drops off.
Its just the repetition that helps.
If she's a grumpy sleeper, she may find it helpful to have cues that signal to her 'Oh, its sleepy time now'- in fact you can say that to her.
eg- getting sleepsuit on... "here's your sleepsuit, lets make you confy for sleep time" ... having a song before bed- "here's your lullaby, ready for bedtime" etc etc
DS is 13 weeks, but I have done the repetition thing from him being 4 weeks or so... I found my older child really needed all the sleep cues as she found it so hard to transition from waking to sleeping... still finds it hard, but she knows when bedtime is, as routine starts, and it helps her.