In my experience it is a bit more tricky when the child is older, say 3 or 4 as they have more fears and it can become more emotional but it is still possible if done in the right way.
Children can be potty trained at an early age (18mths+) even when they cant really talk. I dont really think you have to wait until they are ready as some children will never seem ready, others will show they are at some point. The most important thing is the parent needs to be ready to do it!
Its best not to use pull ups as they confuse children and it feels just like a nappy to them.
First thing to do is to make sure you are calm and relaxed about the whole thing. This will help the child to be relaxed too. Then put pants/knickers on them, no pull ups. You could go and buy them together and make it fun, let them choose.
Then just have the potty in the middle of the room. You will need to give them loads of attention and play with them a lot. You could get teddy and put him on the potty and then put some water in it saying 'look what teddy did in potty, teddy gets a treat now,' and give teddy a treat.
You can ask if they wants to sit on it and play with some toys or read a book while they do. They shouldnt really sit there for too long, it doesnt matter if they dont do anything in it for a while just praise for sitting there. Its best not to ask them to sit on it too much as it will put them off.
When they have an accident in pants/knickers leave them on for a short time then say 'never mind next time you'll get it in the potty.' Dont get cross or annoyed at all.
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