There are a lot of different ways to potty train. Since your kids run, cry, and scream when potty time comes, something's not quite right. A lot of the time it's a little thing that we don't even realize, but the kids pick up on it.
For example, maybe you get stressed out, and they pick up on that. If they know you're stressed, they're more likely to find potty time scary. Or maybe you're offering too many rewards or begging too much. That can give kids the impression that potty time is a chore.
So make sure you take a deep breath and stay calm. There is no need to rush it. Don't beg or plead. Try to make potty time as fun and relaxed as possible.
As Nikolita said, there are lots of articles on the internet, so feel free to search around for tips that you think will work well for you and your kids.
You can also talk to the doctor for tips and to make sure nothing's medically wrong. For example, many kids are afraid of potty time because they have a urinary tract infection, diaper rash, or something else that makes it physically painful for them.
Here are some articles that might help:
This one is about making sure your child is ready:
http://www.nickjr.com/parenting/parenting_features/potty_training/advice/potty_training_readiness.jhtmlThis one is about how to make potty time fun:
http://www.nickjr.com/parenting/parenting_features/potty_training/advice/blues_potty_training.jhtml