Simply load up the campaign mission 'Severed Ties' and proceed until Command informs you that you have enemy helos inbound. Once you have spotted the helos you must change your weapon to the guided round and launch them at the helicopters one by one (you may detonate the round next to the chopper, it does not have to be a direct hit. As long as the chopper blows up you are fine) and each round must result in one chopper destroyed. You can revert to the last checkpoint if you fail as it is just before you engage with the choppers.
I created this video to help:
Hope this helps.