With my two, after a few aborted attempts at potty training DS which ended after 2-3 days of me having to constantly clean up wee and poo from the floor, he went back into nappies, until he asked to go by himself.
Ds asked at 2 years 8 months, and off he went, and never had a single accident, he was in nappies for maybe a week or so after, just in case, but he would remove them to go.
We waited again with DD who was 3 and a half before she was ready and asked to go, again the same thing, kept her in nappies for a week or so, to be on the safe side, but she went fine, and pulled them down like knickers.
Never had an accident at all with either of them after they were ready and asked themselves, and they were both dry at night 6 weeks after day time dryness.
Easiest thing in the world that was, and I had been dreading it. DD was a bit older then I would have liked, but we could have forced it, and said she was potty trained, or been "training" for months with daily accidents, which seemed silly, so we waited till she wanted to do it.