This comes naturally through the story. I would say this is past half the story and 3/4 of the story. A mission will pop up where you have to talk to Martin Madruzo (I think)(Mexican Mob Boss) He will tell you to take out a jet with a person carrying files that he needs. You will play as Michael. Drive up to the observatory and there will be a van with a gun in the back on the cliff. Get in the van and take control of the gun. When you see the jet, make sure to shoot in the red box or you won't hit it. Once you have hit it 3 times, you will switch to Trevor on a motorbike, keep following the jet. You will be in Blaine County. When the plane crashes, kill the guy that comes out. Once you get the files, it should switch to Michael. You need to leave the area and then destroy the van. For me, I had a gas can on me so I poured gas around the van and lit it up. You will then drive to Martins house but Trevor calls you and plans change. Go to where Trevor is. A cutscene should pop up. After that, it should pop up.