First we must train mom and dad. You are the ones who must take and/or remind for the first few months of the training.
Rewards are good. It must be something that is only given if the child goes potty. If there is an accident don't be afraid to let your child know you are disappointed but he/she can do better next time.
Praise works wonders. We clap and say what a big boy/girl. Praise them in front of other people or at least talk and let them overhear you.
You will have to take them often to the potty at first until you find the times they need to go. Each child is different. Some go every hour and others it seems very seldom.
This is a long process don't expect overnight success. This can and usually takes months or your reminding and/or taking. At play time they don't want to take the time.
Sometimes I let them sit on the potty for up to 10 minutes. I also leave them alone checking on them every so often. If they haven't gone I say "Maybe next time." (Do not give a treat except when they go.) Next time will be within the next 1/2 hour or maybe even sooner.
One of the most important things I have found is do not use pull-ups. These feel exactly the same as a diaper to your child. Use training pants. These are the padded underwear. If they wet leave the wet ones on for a while so they can feel that being wet is not a good feeling.
This is just a few pointers I have found to work over the years. I understand this can be a long and trying process. GOOD LUCK!!