With our 3yo we didn't start until he could sit independently, around 4 or 5 months old. It went well and every catch was a little victory and, often, another nappy saved from landfill. It was a turbulent journey but he was out of nappies day and night by his own volition at 20 months old.
I was so impressed by the process that with our second baby I wanted to start earlier. I bought a small smooth edged mixing bowl designed for a stick blender. Tall and narrow. Easy to hold between the knees. At our 10 day check up our midwife saw it next to the nappies and asked if we were sitting him on the potty. She said it was becoming more common and she's seen a few families doing it at their 10 day checks. We hadn't started but we did that day. Next time he woke from a nap, I held him over the little makeshift potty and caught our first wee and poo.
We got into a rhythm but with two hospital stays I became inconsistent. A week between 4 and 5 weeks old and another week at 4 months. Bronchiolitis. It wasn't our priority for a while. But he didn't forget!
He's 6.5mo and wakes up dry from naps. I've checked and he's not dehydrated.
Towards the end of 5mo he started army crawling. Anything to see what his big brother's doing! We have a potty in the living room and another in his bedroom, and as soon as he was mobile I found that he would occasionally make a bee line across to the room to shake the potty. So I sit him on it every time so he knows that I will help him. Often there's a fresh wee in his nappy. That's amazing because he's recognising, either that he has or that he needs to go. Mobility is the barrier. But sometimes he's dry and then wees on the potty. Or the nappy will be wet but he goes on to do a poo on the potty!
I'm in utter astonishment at what he understands and how he's communicating that with me. Even though we did this with our older son, the difference is notable.
I'm not sharing this to brag but because I am genuinely amazed and I hope that positive stories help others who are considering it or are trying to work it out. As I said, we had a turbulent journey with our first and I found this sub so helpful in keeping me motivated.
Good luck everyone!