How do you put a baby on a schedule?!

AB
This probably seems like a stupid question, but my 15.5 week old has a routine, three naps a day, etc.  But things almost never happen at the same time on the same days. I understand how it would be easier to feed baby at the same times everyday (I'm EBF), but as far as getting baby to sleep at the same times every day, how do you do it?
Our baby girl fights sleep and was a decent night sleeper until the 4 month regression hit, but she requires an elaborate bouncing ritual to go To sleep (nap or nighttime sleep) and fights it very time.
She also never seems to get the daily recommended 15 hours of sleep for her age, so I never want to wake her To try to adjust her schedule because I want her to get As much sleep as she am get. But should I be waking her? And if I do wake her to say try to have her wake up at the same time every morning, how many days until I can expect that things fall into line? 
Also, if baby is on a strict schedule, how do you deal with things that throw a wrench into the routine, for example a doctors appt. Scheduled during a time baby is usually napping etc. ?
Thank you!! Any input or help would be so appreciated.

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