proper way to suck balls?

My husband always tells me "don't forget the balls" when I give him a blowjob😂 I'm always super scared because their balls are so sensetive and stuff. So when I do, I'll lick around and suck on them and he will say "harder" like what do I do harder I don't wanna hurt him! What is the proper way to make a man feel good with them balls😂😂