My newborn daughter was, I guess I could say, self trained. There were times when she cried and fussed at night. But what I did to make her sleep again was very simple. I never talked to her or sing or look into her eyes. According to an article I read few months back, or maybe years, it says you shouldn't look into your baby's eyes when trying to put him/her to sleep cause it will keep them awake. Not sure of the explanation anymore. Also do not talk to them cause they will take it as a play, or that they have to stay awake and listen to you. Singing it's optional, my daughter just didn't like to sleep w music. She would listen to different songs, not a lullaby, and then I would have to turn off whatever device was playing or she would fuss and start crying.
Further more, daytime play w you baby, just don't overstimulate him/her which will lead to crying. Observ when your baby is sleepy and try to put him/her in a crib or bed to settle and eventually fall asleep.
Also, do not turn on a big light in the room, have a lamp or smth small to generate enough light for you to see if changing diaper or feeding. No tv as well 😏.
It might not work for everybody, but I just wanted to share my experience.
Good luck to all parents!!!