I waited until my DC was showing an awareness of needing to wee/poo, e.g. pausing in their play right before doing it, pooing at a similar time each day, indicating to me that they'd done something in their nappy (verbally or non-verbally), etc. They all started doing this between the age of 2 and 3 and a half.
Once they did this I took them to the shops to choose some underwear and then picked a time when we could stick close to home for a few days. I trained straight onto the toilet with no potty and no little seat either because when you're out and about there won't be a potty or a little seat.
Day One, no pants and regularly asked them to come try the toilet. Lots of praise for just trying
Day Two, pants on and regularly asked to try the toilet. Lots of praise.
Day Three, pants and clothes. Reminded about trying the toilet. Lots of praise.
They're not going to get it perfectly right and there will be accidents but there should be some noticeable improvement day on day and by day three there should be more successful toilet trips than accidents. If there was no improvement and if the toilet successes aren't out-numbering the accidents then they went back into nappies and we tried again after a couple of months (did this with one DC and attempt row went much better).
By day three, all of my DC were reliably trained enough to be able to go out and about in pants. Tactical wee before leaving the house, tactical wee on arriving wherever we were going, and then little reminders across the day as needed then these gradually tapered off over the first two weeks.