Here is a simple tutorial on how to make doll joints and how to use them.
What is the advantage of doll joints in general?
They make the limbs of your doll/plush movable which is really cool if you're going for realism.
These are cheaper than the ones you can buy because it's quite likely you will already have all the materials at home.
They are however more difficult to use.
Please make sure to use wire that will not rust. If you had it lying around for years and it still looks fine, it will be okay.
I can not guarantee that the joints made with this tutorial are safe for toys for children.
Feel free to ask for advice, point out mistakes, or leave lovely comments.