It akes time! Don't stress about it! Kids don't get these things the first time...you have to keep repeating over and over and over again. It also helps to have some older kids who understnad and can model what you want them to do. I wouldn't stress about getting them to sit. Also, do you have any playgroups near you that you can take them to? I find the kids sit much better in a large group where there are many other kids sitting and modelling the behaviour.