Stupid question about riding my boyfriend...

How do you ride him by only moving your ass? I know it's dumb and I've watched videos but idk if I can move my ass like that. Tips? Anything? 😂 I ride him and usually use my legs but he said I should be able to by only moving my ass. 
Edit: Like grinding on his dick while sitting on it? I'm 20 but he's the only guy I've had sex with and I'm awkward AF. 😂 
Edit: Okay, so, grinding on his dick while sitting straight up HURTS SO BAD. Maybe because he's big and I have a tilted uterus? He loves it and it feels great for him but OMG. Pain advice?