This filter type dic fits on the front of your lens, snapes in, there are on threads. Once on the camera all that is need it to set a custom WB with it pointed at the light you will be taking the picture in. It has a nice string that will go right around your camera lens, you can use it as a lens cap.
It works very well and does a real nice job of getting the WB correct. It is somewhat high in cost but if you are taking pictures in all different light I think it is will worth it.

Link to Expo/Disc site.
http://www.expodisc.com

By Bill Huber