Duotone balls are primarily meant to be used for kegel exercise. Put them in your vagina and clench your muscles around them - simple as that. As kegel exercisers, this set is large and lightweight enough to suit a beginner. Gravity trying to free them shouldn't be as much of a problem as it can be with smaller and heavier balls.
Okay, so this is a good size to get inside yourself and squeeze. Good workout, but wait! That's not all! Duotone balls are hollow and have weights loose inside of them. I love this feature so much - I can wear them during a workout or for everyday activities and can feel them moving around in there. If I an going up or down stairs or bending over or any one of a hundred other normal movements I can feel those weights shaking to give me a pleasing internal stimulation. I enjoy this way more than I do having solid balls inside me! It's especially fun for running errands since no one knows they're in there but you. All that moving and shaking may not bring on an orgasm by itself (it didn't for me), but it sure got me wet and excited.
These are not safe to use anally. Strictly vaginal, folks!

Duotone balls are made of phthalate-free plastic. The cord is made of nylon.
The plastic is smooth, firm, and non-porous. The smoothness makes these easy to insert, especially if lube is added. The cord is soft and silky. Beginners should have no trouble with these guys.

I like this design for the most part, but it could be improved. The balls themselves are a great size for me, and the way they are weighted makes them highly pleasurable to have inside me.
My two complaints will keep me trying out other balls though:
1) The seam around each ball is a minor annoyance. I couldn't feel it once I had these inside, but it just screams "you bought a cheap toy!" - it kind of makes them look flimsy too, though I doubt they are.
2) The cord. What is up with that cord? The length in between the balls seems unnecessary, and the "easy grip loop" is so short it disappeared inside me as soon as these were in. I'd prefer no cord at all.

The actual measurements are:
Circumference: 4.5"
String connecting balls: 3.5"
Retrieval loop: 2.25"

Here they are compared with my other set of balls, a lipstick, and a quarter:

My two minor quibbles with the seam and the superfluous string did not get in the way of these guys working well for me.
I cannot state enough times how much I love the way these are weighted and the motion that gives them. I love it, I love it, I love it . . . .
I like to keep balls in during sex, and these are a mixed bag as far as that goes. Since they're bigger than my other ones, there's not as much room. On the other hand, my partner said He could feel the motion from the weights, and that it added some extra stimulation. That's worth a little crowding!
As kegel exercisers, these are useful to me. I can really squeeze around them since they're bigger, so I'm hoping to move down in size and up in weight as I get stronger. Having something to squeeze against can make kegels turn into pure arousal pretty fast for me, I've found. No complaints there.

Plastic is easy to clean and care for. I wash these with antibacterial soap and warm water.
I store them in a smaller box I keep inside my toy chest. I'm careful to put them directly into their box after cleaning, since if they're left out they look just like cat toys. (Naughty kitty!)
Any lube can be used with these balls - water, oil, or silicone based.

Oh, California Exotics, your packaging always makes me feel like I need to put on a trench coat before I buy anything. I've never been a fan of their packaging as it always looks cheap to me. I do appreciate that they have a lot of affordable toys - really appreciate it since I'm on a budget - but I feel like they could class up the photography and product copy a bit.
This one has the usual sexy model (or rather, the bare back of one, with a string of pearls hanging down) and the product name, with ORGASM in huge letters. I do love orgasms, but that's a slight overstatement since I doubt most women can just slide these in and start coming wildly.
It also says they're "scientifically weighted for your pleasure" - a feature I loved, and I am all about science. Good to know it's working for me.
The insert and balls are in clear plastic clamshell packaging.

Overall, I do think these are a good value for what they cost, since I like them WAY more than the other set I have [https://www.edenfantasys.com/sex-toy-reviews/female-sextoys/balls-of-steel] which cost almost twice as much. But now that I know how much I love these I'm ready to invest in something a little more high-end.
Now I know I like this size and love weighted rather than solid balls. More research is needed!