When you land take the train to the Baggage Claim area, turn toward the DELTA baggage claim at the top of the escalator and follow the signs to Rental Car Center. You will take a right in front of the Delta baggage claim. You walk past the end of the baggage claim on the South (Delta) side. This is called the West end of the airport. Walk straight past the MARTA entrance and go outside to catch the Sky Train. It will say "Rental Car Center and GICC Gateway Hotels". That is the SkyTrain. You get off at the first stop for the Marriott instead of the second stop for the Rental Car Center.

The next day just reverse your steps. You will arrive inside the terminal building near baggage claim, so keep walking straight past all of the South Terminal baggage carousels. Then Delta ticketing and baggage check will be on your right, or main security (and the Clear with PreCheck) on your left across from the SkyPriority check-in. If you need to check in at another airline, you need to make a left at baggage claim to the North Terminal (which is really the other end of the same terminal)

After security, Concourse C is about 8 minutes on the train, max. On a Saturday AM, I will walk out of the hotel around 75 minutes before my flight, and I have time to visit SkyClub.