D.K.
We trained DS when he was 25 months. We just put him in underwear, told him pee and poop went in the potty and took him to the potty frequently. We did it on a weekend so we did not have to leave the house much. We did not do rewards, he was really happy when he successfully used the toilet. We did read Tari Gomi's book 'Everyone Poops' a lot while he was on the toilet (this caused us to spend an inordinate amount of time in the bathroom). He had a few accidents over the first week and then one or two over the next month. We had tried naked all the time for a day before underwear - he was perfectly happy peeing on the floor so we gave that one up right away. We wasted some money on pullups for a few days - he had no hesitation about peeing and pooping in them so I think they were worthless. We have friends who used the cheerios method to teach them to pee standing up.