Depends on what you have (sort of) For a PC just push F9..(push F12 first if you want to get rid of interface)
If you have a mac you may have to play with the settings first to be able to do that:
1. Open System Preferences
2. Look under Hardware; Keyboard and Mouse
3. Check: Use F1, F2 etc. as Standard Option Keys button (This just makes it so you don't have to push fn. If you leave this unchecked it's fine. Just make sure to push fn and F9 at the same time when taking the screenshot)
4. Next click the Keyboard Shortcuts tab
5. Scroll down and uncheck F9 (F1-F6 for weapons, F7 for game options, &/or F12 to get rid of the interface)
Now if you go on game any of the F_ options you unchecked should work as normal function keys.
Sorry if that is a little confusing. If your still having trouble I will try to explain it better....
