In the wave options when customizing a firefight game.
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I don't believe you collect any. You just turn them on or off.
well it depends what halo it is, if it is ODST then it should be simple and you read the manual and follow those instructions or do not sign in another controller. for REACH you just turn off all controllers sign in and it should take it from there If you are speaking of playing in Reach's firefight Solo, there is a selection in the main menu called "Firefight" choose this rather than going into "Matchmaking".
well depending on what skulls you turn on the skulls will make it harder [usually] or more fun [only two actually make it fun] but will improve your skills and give you more points
no
no you can not but there isn't that much really