This mission can be gotten from Trevor's house, so head over there. Hop on one of the quads and head over to the local Ammunation, to pick up a sniper rifle with a suppressor and advanced scope (all free). Get back on the quad and meet up with Ron nearby. Climb to the top of the water tower and get ready for some sniping.
Start by getting a read on Ron and then follow him until he stops. He will point out that there's a guard standing at the bottom of the stairs under the control tower. Ideally, you will want a headshot, but a body shot will also kill the guy in one hit. Another guy will be arriving soon by car, so Ron points out to shoot out the two lights overlooking the bottom of the stairs.
For now, just wait until you get the prompt from Ron to shoot the guy in the van. Next will be a guy coming out of the control tower, so ice him, then one approaching the one on the ground and another in the control tower. There will be two guys coming out of the building near the gas tank to the right, so kill them and Ron will move to plant the bomb.
Another guy will approach on a motorcycle, so when he stops, shoot him and then a helicopter will approach. Wait until the chopper stops moving, then get the pilot, which will alert the rest of the enemies in the area. Get on your quad and quickly move to the area with the gas tank. Get into cover and start taking out the bikers, remembering not to stay out in the open for too long.
Once you clear the area, get to the plane in the hangar and Trevor will jump onto the wing. Note you seem to have an unlimited clip in this segment, so keep firing at the bikers while the plane moves to the other plane in the area. When you get the prompt to detonate the explosives from earlier, do so. You'll automatically jump off and get into the other plane.
Start accelerating and once you have gotten enough speed, pull back on the left analog stick to take off. When you get some air, start moving the plane back and forth to get the guy off the wing, then just follow Ron. When you get close to the buyer, which you can see by the smoke signal, you'll have to fly low to avoid detection from the nearby military base.
Once you get into range, press the designated button to drop your load near the buyers. It's easy flying the rest of the way back to the hangar, although you will need to land the plane. As you're approaching, ease up on the accelerator and get your landing gear back out. Slowly make your way to the ground and hold down the brake once you touch land. When you land, taxi the plane into the hangar to finish the mission.