Book a tour on the website mentioned below. You can either choose a morning trip with a lunch after your visit or the evening trip with dinner included. You will be picked up and dropped off again by a small bus.
At the sanctuary, first you get the chance to pet the elephants and get to know them a bit. The staff will give you more insight on the behavior and (cruel) history of each elephant. In this way, the locals also try to explain to others how elephants are mistreated during tourist pleasure rides or in a circus. Afterwards you can feed them and touch them while doing so. After their lunch or dinner, the elephants are offered a nice and refreshing mud bath. Get in the water and help them cool off!
I must admit that I am usually not into these kind of things where they keep wild animals behind bars so I was a bit skeptical at first. The elephants remain, somehow, a tourist attraction but, on the other hand, if you hear the individual story of each elephant they are much better off in a sanctuary and are truly pampered there. The staff even explained, they do everything possible to release elephants back in the wild but in most cases it is just not a good idea.