TT is very doable with just undies on. At daycares for example, when they start tt children, they put them in undies and take them to the toilet every hour. They have them sit there for as long as possible. You do get quite a few accidents, but you have to be ready for it. Unfortunately there's no way around that. They need learn what doing wees and poos are, and a lot of the time they need to do one in their undies, or on the floor to realise. When we started tt Nina, she did poos on the toilet just fine, but wouldn't do a wee on the toilet for about 2 weeks. It was a nightmare, as there were lots of accidents. I was persistent because she was showing the signs of being ready, but just too scared to do a wee on the toilet. Once she did her first wee on the toilet, she never went back.
Each child is different. Just wait until you feel she is ready. Don't worry about her age, she could be 3 and still in nappies, that's ok.