How do you use a CatapultFollow

I tried for the first time to use a catapult in bg3 last night. I was a disaster. I figured out how to do the ground target but for some reason I could not load the stones into the catapult and when I moved away from the catapult my option screen disappeared and I could not get it back. Can anyone out there give me an indepth discription on how to use a catapult. Thanks, Loron