how do you use the camera thing on halo 3?

[Cloud]

i did it once but i forgot how to d it. i did this cool plasma kill and i didnt turn it off so i want to know how to do it

 

[Arnold]

Go into theatre and play the match in which you did the thing you wanna screenshot - zoom up to this thing and the press (X) to open up the menu and press take screenshot - It will be saved on your list of screenshots in the theater