Follow the adoption laws and requirements of the birth country, then get an adoption visa for the child to come to the USA. Get a US immigration attorney for help.
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Adoption between the U.S. and Mexico is governed by the Hague Adoption Convention. To adopt from Mexico, your must first be found eligible to adopt by the U.S. Government (the agency responsible for making this determination is USCIS). Furthermore, there are residency, age, marriage, and income requirements. Contact an immigration attorney.
I have reposted your question under Immigration, for more input.