While normally the guy is behind the girl, feel free to switch things up. The girl's hips should move in a circular motion, and the guy's hips should match that motion. Before the guy puts his hands all over the girl, the dancing pair, or trio, should find a comfortable rhythm.  If you're in front, move your booty around in a pattern at around the crotch level of your partner. You're in control. If you're behind someone else, follow her movements. Your pelvises can be directly aligned or slightly off so that of your legs is between your partner's legs. Once you found a comfortable flow, the guy should put his hands on the girl's hips. He doesn't have to grip them too firmly -- just enough to gain his footing and to settle deeper into the grind. The girl can bend her knees, so the couple can move lower to the ground.  As the girl gets low, her hands can travel down her thighs and even to her knees. The girl can keep her arms loose. She can move them up and down to the beat of the music while grazing her own thighs or knees. Guys should expect a booty shake, and girls should expect to shake their booties. Gradually, a girl will get lower to the ground, putting her hands on her knees, or holding the guy's hands near her waist. As she leans forward, her backside will raise in the air, brushing up or hovering near her dance partner.  The guy's hands can travel up from the girl's hips to below her waist, at the top of her backside. This will work as long as the guy is respectful and the girl is comfortable with this movement. If you're up for it, the girl can get even lower to the ground until her hands are nearly touching or touching the floor and her booty is raised high. Sometimes a group of people start grinding together in a chain. Usually, you'll see either a mix of men and women alternating or just women. Sometimes two women will "sandwich" a man, and others will latch on. Don't hesitate to go with whatever makes you feel good. Grinding in a group can be even more fun than grinding with just one partner, but...

TL;DR in Chinese:
以圆周运动移动你的臀部。 男孩应该把他的手在女孩的臀部。 降低位置。 群体热舞。