I sleep trained mine, both of my kids have been good sleepers since the very beginning though.
My son finally slept through the night by 3 months. We used the "reduce-the-milk-amount method, he was already sleeping from 7:30 pm all the way to 5 am, by 3 months he was finally sleeping til 8:00 am. I always put both of my babies fully awake in their cribs when I knew they were ready, I was very consistent no matter what, I knew he was going to get it sooner or later.
I was finally able to put my son fully awake without pacifier by 4 months. Starting with naps, I always put him in his crib when I knew he was ready, he cried the first few days and then he did it. But I had to do that every single day at the same time for a very long time, until he finally got it.
He has a very good sleeping pattern. Sleeping 13 to 14 hours straight and having a two hour nap around noon. He's able to self soothe and put himself to sleep.
My daughter is almost 3 months. Tonight was the 3 night in a row she didn't wake up for her night feeding. She goes to bed at 7:00 pm and wakes up for 2 oz at 4:00am. So we're trying to get her to sleep thorough the night. She's done it before, around two months but started waking up again a week ago. I go in her room and give her the paci for 5 mins and she falls back asleep. If she is really hungry I'll know she won't think about it before asking for food. She finally wakes up at 7:00 am.
Some babies are good sleepers since the very beginning, mine were, so I just guided them, I actually didn't have to work too hard. But I find that following a constant schedule helps a lot. It won't work 90% of the time at the very beginning but don't give up, it'll click sooner or later. Find out when your baby's wake up time is during the day and start with naps first. Once her daytime schedule is set, the night schedule is easier. I don't let mine sleep more that 3 hrs during the day, and no sleeping past 3:00pm. That's why they sleep so much at night. Whenever I break my own rules they wake up a lot earlier than they usually do. Every time. So I figured this is what works for me. Find what works for your baby, let the baby tell you!