Instructions
Locate the bomb. The bomb icon will appear in either blue or red, depending on your team's color. (For example, if you are on the blue team, your bomb icon will appear in blue.)
Press "X" to pick up the bomb. (Although "X" is the default action button, the correct button may vary depending on the controller layout you are using.)
Take the bomb to the arming station. Your team's arming stations are also color-coordinated to match your team's color.
Arm the bomb by simply standing inside the designated circle until the red bar at the bottom of your screen fills up and the words "Bomb Armed" are announced. Once you have armed the bomb, move away, or the explosion will kill you.