If I have someone have a baby for me how does that work

I assume you mean a surrogate mother. Generally, you enter into a contract with a woman to carry a baby for you. Either one of her eggs is impregnated (in a lab) with a sperm from your sperm donor, or one of your eggs may be implanted in her after being fertilized.

You pay all her medical bills, and after birth the baby is given to you.