I emphasized the word plastic because you're not making real plastic. It's a substance that feels like hard plastic. Watch this video and you'll see and hear that it's like hard plastic.
I suggest you view this guide as a fun science experiment rather than a really useful substance. Although small things can be made out of this substance. Its safe enough to do with your kids too.
Poor some milk into a bowl. How much will depend on how much you want to make. But just to experiment with. Use about a cup and a half to two cups of any milk (I used 2% but it doesn't really matter).
Microwave the milk for about a minute. You don't want it to get hot enough to boil but just hot enough that you'll see a little steam rise from the bowl when you remove it from the microwave.
Now pour in a table spoon of distilled white vinegar. Then slowly stir it. Right away you'll see the milk will start to curd. As you can see we're basically making mozzarella here.
This is similar to how to make mozzarella but this stuff won't taste very good. :)
Gently stir this around for a few seconds. Just to give the acid in the vinegar time to do its job and curd the milk. It won't take long and it'll look something like this.
Strain the mixture and you'll be left with this goo in your strainer. Get as much liquid out of it as you can and let it dry on paper towel for an hour or so.
This stuff isn't like clay or play doh. It's more rubbery so it won't shape as well as play doh. But you or the kids can shape it any way you what to. Just don't expect it to be to pretty. :)
This is the same disk i used in the video in step one after I did a little sanding on it. So you can sand it into shape to make things look better.
You can put it in some kind of mold. Here I'm making a cube from a cardboard mold. Let it dry for a few days. Or better yet for a week. It'll become rock hard and you can sand it into shape.
Like I said. This guide isn't about making something really practical. It's more of a fun project or experiment to play around with. I hope you found it at least a little fun.