You don’t start with “Let’s sit on the potty and poop.” You’re going to end there, but that’s not where you start. A good place to start is with a potty chair, not the adult potty. The reason for that is twofold: Number one, sometimes kids get intimidated by being up there on the adult toilet. It’s like you sitting on a ten-foot platform. And two, it’s very hard to poop when your feet are dangling. If you do use a little thing you put on the toilet seat, you need to have a little platform or step stool that they can put their feet on.
The first thing you do with a potty chair is you get them sitting there comfortably like it’s no big deal. You do that by making it more a party chair than a potty chair. The child will sit there, fully clothed, just to read a book or to sing a fun song. Then, “Come on, honey, let’s sit on your magic chair and look at a book. Which book do you want to look at?” When they’re sitting, set a timer for a minute or two. When the timer goes off, you say, “Yay, that was fun! Bye-bye, special chair. We’ll see you later.” The child is going to go, “Wait, I’m not finished. We were reading a book.” And you go, “Okay. Yeah. We’ll finish this page, but then we’ll come back and read some more later.” Get them wanting it more than you want it.
Once a child will do that, the next step is to do the same thing but without pants on. The next thing is to do the same thing but without a diaper on. Then, oftentimes, whether by coincidence or through their figuring it out, they pee or they poop.
I’ll note that it’s a good thing for boys to learn to pee sitting down. Because once they can pee standing up, they don’t want to sit. Standing is more fun. So getting them to learn to sit down is helpful, because that will help them figure out how to poop sitting down faster.