Had a southwest flight in terminal 1. From security if you walk to your left and head towards the baggage claim for terminal one you'll see the trams for d gate. Take the d gate tram to the d gates and head up the escalators. The centurion lounge is pretty close by.
Even though it sounds like an ordeal in actuality probably took me about 10 minutes after walking through security to getting to the lounge.
If you're wondering, you don't need to go back through security going from the c gates to the d gates.
Food was only decent at the centurion lounge but they had a wide variety of drinks. Definitely worth the trek.