How to go to Huanglong?

It is not necessary to book a tour, because you can travel on your own.
From CD Xinnanmen Bus Station, you can find the buses for Jiuzhaigou. You can take that bus to reach Chuanzhusi where you can find frequent buses to reach Huanglong.
I think it is hard for you to visit there and back to CD city in one day, because the place is a little bit far from CD.
If you want to book a tour, youn can contact TravelChinaGuide by sending them email at travelchinaguide.com|tours.

I prefer early preparation of seeking information to go there. The Chinese there are very helpful, but there might be a bit hard to communicate because very few of them can understand English. I would love to go there, I love China and it's beautiful history very much. Thanks for your assistance Edgar.