International flights arrive at the International Terminal.

You will have to pick up your bags and clear immigration and customs at the International Terminal. ATL immigration has been known to be slow.

You will then head to the domestic terminal to check in for your flight. The problem is that the both terminals are linked by a train airside. Domestic and International Terminals have 5(?) satelite buildings between them. The train stops at all satelite building.

I am not sure if there is a train that services landside passengers. You have have to take the shuttle bus. Are you on a flexible DL ticket? If so, book on a later flight and see if they can move you to an earlier flight if everything goes smoothly.

My first time at arriving at ATL with a domestic to international transfer have done no homework, it took me 15 mins to work out the set-up and where to go.