the fire button brae XD
You don't have to press any trigger. You should automatically receive the achievement.
When you have the skull menu brought up, click on the skulls you want to activate to activate them. Simply, move the control stick to the skull, and press the button.
you press the L button
You have to press the Lt button.
Press the Start button.
press x button
You just go near the skull and press RB like you would pick up a weapon.
In Halo: Anniversary, you press the 'back' button on your controller to switch to the old graphics. It is left of the Xbox Guide Button. You can only do this in Halo: Anniversary.
Its not an achievement, its a silver skull. On the mission "Crows Nest" (the part where the bugs are coming from the pipes after you drop down and Cortona talks to you, at the end at the edge), you will notice that you can go underneath the walkway you were just on. The skull is at the back of that passage.
press the triangle button
push the y button to swicth weapons and the skull should drop.
When you go on to the campaign page press (X) and then select the skulls that you have collected.