From my memories, and the accounts from my parents, I pretty much just switched from diapers to underwear. I have one memory were I was three and there was a baby-sized toilet on the floor of the kitchen and I used it. I felt very vulnerable, but after using it, I don't remember much.
(Just for fun) In the same setting, in the kitchen in our house where the potty originally was, my mom and I had a conversation; I think I was four, about Michael Jackson and how he bleached his skin. Crazy, right? That was like 1987 or something.