I'm a US citizen going to Nigeria to marry my fiance what is the next step to get him to come back to the US with me?

Hi. We can process her green card when you return. There are some specific items, documents and signatures, you should come back with. The process may take 8-12 months. The timing depends on how busy the Consulate gets. The case is first filed in the US and then forwarded to Lagos about 8 months later. Let me know if you would like further assistance.