I'm struggling with potty training for my 18 month old girls. if I try to make them sit in squatting position or on seat, they start crying. I am unable to understand what and how to begin.

try making it fun. use a baby potty training seat that has cartoons. put music on....make it fun

Hey Shalu...don't worry go with the flow. Rush to the washroom only when they are feeling like it badly and can't control. They might be getting a feeling of Fall on the potty seat otherwise. Btw please read the article On tips for Potty training. it has some practical ideas and I found them helpful