The easter egg is a long walk from the start point on the map, so you will need a vehicle. It does not matter which vehicle (tractor, DLC gator, or truck).

Here are the steps:

1: Hop into your vehicle and head for the middle of the map.
2: Around field 16 (I think?), you will see a wooden windmill on a cliff, and a medium bush around a shaded area.
3: You can either hop out of the truck by the bush and walk into the secret area, or drive up to the windmill, go into the open door, and head down the trapdoor (both ways lead to the same area).
4: Go take a rest. You earned it.