It takes practice. You wont be able to read something on here and learn how to grind like a pro.
But basicly theres going to be a beat to every song, some are fast and some are slow, either way you need to move to it in rythme. It takes a while to be able to pick up the beat and move to it smoothly.
My first dance 3 years ago was so embarrassing. I was akward and couldn't stay with the beat.
Now 3 years later and a handful of dances under my belt I have girls telling me that so and so cant wait to dance with me because im a "sexual dancer"
The best piece of advise I can give you would be to try listening to music you think they will be playing if you dont already. I listen strickly to classic rock but when a school dance is coming up I go out of my way to listen to some recent pop music. I dont like it but it gives you a great advantage to be familiar with the songs that they will play.
The thing I think most guys mess up on is they are afraid they may violate the girl. They keep their hands the the girls hips and are careful not to be too forceful. Instead of being a faggot whos afraid to control a girl, have your hands wander from her hips to her stomach to her ass and all sorts of other places. If the girl is a good dancer I like to sometimes lean back slightly and place on hand on her lower back, gently forcing her down from the waist. Thats when they get crazy with their ass. Dominate sh1t like that, thats what girls love.