You can go there on your own or book a tour (both in Sapa and Hanoi). The fields look amazing all year round, during harvest and even winter when they are covered in snow.
It's a big building where you can walk around and you pay at the entrance for a specific activity. There are some discounts and packages if you're interested in going for a few. Wax museum wasn't good, the figures aren't authentic, but odditorium was very interesting.