I hate to say this, but my kids pretty much trained themselves. Three of them were two or younger and just wanted to use the toilet, so they did. I did try to train them when they were younger and it just didn't work.
Our daughter was about 20 months old and kept bringing diapers to me (I thought because she was wet) but she was dry. My mother said she needed to use the potty, and that was what she wanted. She wore diapers at night for about another week, and that was it. Very fast and unexpected.
Our first son was 26 months and the second son was about 3 or maybe a bit more. He copied his brother and did it himself. They were all dry overnight by age 2, but I kept the diapers on to be sure.
Our fourth son was much younger than the others so he was very aware of the toilet at an early age, so was trained by 2. I didn't do a thing. It took longer for him to be dry at night though. Probably by age 4 he had stopped.
My pediatrician said the worst mistake parents make is to try and train too early. He always said they will let you know and they did. I had friends who started training their kids at 18 months and it took six months to do the job. I can truthfully say it was a one day shot for all 4 of mine.