Listen don't be hard on yourself. Balls are an insane thing.
I myself own two and barely know what the hell to do with them. As idiosyncratic as dudes are with their wang, they are double when it comes to the nuts. That being said, there are two pillars you should know and breathe when handing the balls. The first being a cornerstone for both Doctors and ball handlers: Do no harm. If you don't know what to do, do nothing at all. A scrotum is stupid sensitive. All those '80's movies with guys getting socked in the nuts come from a place of truth.
Squeeze, press, poke or pull even a smidge too rough and the nuts will let you hear it. Rather, they'll let their owner hear it and you (the lady) will be subject to man's primal survival mode. You'll be choked out, thrown off the bed, or hand slapped so hard you'll think you're in grammar school in the 18th century.
Second, ASK your man what feels good. You can't expect to navigate a man's nutsack blind. That's like asking an Amish kid how to get to Brownsville, Brooklyn from Central Park. They need a map, a gentle guide and a willingness to fail. One size does not fit all when it comes to hand jobs and especially ball play.
So there's your answer. I'm sorry it's not more specific, but that's the point: it's on a nut by nut basis. What's lovely is that you've shown you have the interest and gumption to incorporate your man's scrotum during sex play. You're forward thinking, and I'm sure in no time the four of you will have a ball.