My friend had a similar problem. She sat her kid on the toilet (with the nappy still on) until he associated poos with the toilet. Then she'd make him put the poo into the toilet afterwards and soon he realised how annoying and messy it was, so started just pooing on the loo.

If your daughters poos are always soft then perhaps change her diet a little - poos should be firmish, not sloppy etc. (at least that's what I've been told!).

Good luck