How do I treat the rash in my baby’s neck folds?

Ointments can help. Until those baby folds turn into more of a neck, this area tends to get irritated due to moisture, sweat, rubbing, and drool. Keep this area protected with your favorite greasy ointment (vaseline, aquaphor) or diaper cream with zinc. This will provide a good barrier and protect skin breakdown. Clean the area gently between applications and during bathing.
Keep clean and dry. The neck area can get moist. Rashes in moist areas can have yeast, so a doctor should see the rash to decide whether or not anti-yeast cream or powder is needed. If a neck stays clean and dry, there is less chance of a rash developing.