Simply load up the campaign mission 'The Embassy' and proceed through the level until Price orders you to pick up the designator. Now clear out the enemies on the construction site and once looking to the east (your right), there will be a green civillian car that drives by. After that there is 3 of the 4 trucks that you must destroy.
Aim in front of the trucks as you have about 1-2 seconds before your strike is called in but they are generous with the blast radius and it isn't too hard. Feel free to revert to the last checkpoint if you miss a shot. After hitting the 3 trucks you have some more enemies to kill, listen to your squad for when the next truck is coming and then you will have your chance to hit the 4th and final truck.
I created this video to help:
Hope this helps.