Reward every time they sit on the loo or potty. This is not about the wee or poo, but installing a habit.
We sidestepped the potty and went straight for the loo. I just felt that the dcs were able to go for it, and sometimes the transition from potty to loo can be unexpectedly problematic and we wanted to avoid that.
It is a long process and it's all about the process, so be sure to reward every time they sit on the loo and not for results in the loo.
Walk with them from every place in your house to the loo, so that they know where to go.
Set a timer for every half hour and walk with them to the loo, and get them to sit on it. Reward them every time with a sticker. 10 stickers gets a small toy treat. Don't associate reward with sweets. Don't associate the reward with a wee or a poo. It's all about the process of going to the loo, taking down pants and sitting on the loo at this stage.
Be sure to have plenty of fibre in their diets, (or half a spoon of syrup of figs) as sometimes doing a tough poo in the potty / loo can be traumatic and you do not want constipation as it can lead to holding in poo which is a nightmare with its own personalised level of hell.
You need patience, consistency, good humour and a sense of perspective, and a sticker / toy reward system.
Get plenty of gin.
GOOD LUCK!