I was hopeless at this so ,y tips might not be so great
1 - offer rewards. started off with a lolly or lollipop for a poo, and reduced to a smartie
2 - outside play ... either with knickers/undies or nudie
3 - sticker chart
4 - patience. it takes time, energy and plenty of accidents.
5 - I had a rule, after 4 accidents we had pullups