These are the steps which I used and was fairly succesful. That it I was only covered in poo twice, there was wee on one car seat, we had to stop at the side of the road in traffic and my toddler went to playgroup with wee on his trainers but a few days of stress and it worked.....
There is no timescale....pushing they before will be stressful for you and her.
1. Take them to a shop make a big deal of buying the pants *she wants ie princess or whichever,
2. Let them buy her own potty too.
3. Buy stickers from somewhere (poundland etc) and let them decorate it.
4. Put potty in sitting room or somewhere it is seen all the time. (Later move it ...to the bathroom)
5. For today and tomorrow no pants, see how successful it is if it isn't don't push them.
6. Take them with you to the toilet but don't pressurise them just let her see what's happening. If they have an older friend or cousin ask the, to do same.
7. Take a potty with you if going out but would try not to go out.
8. Ask them every 10 - 20 mins if she needs a wee wee etc....
9. Don't scold them Don't stress if it doesn't happen. Stop and try again another time.
10. Reward her with something little if she goes....