From Story Mode, press Options to open the menu and press left. This takes you to Rockstar Editor. Enter this and select Director Mode (you need to be on foot, with no cops on you, outside and not have a wanted level for this option). Pick any Actor you want and it will load into Director Mode.
Hold down on the D-pad to get an option to record. You must record for at least 3 seconds. Once done, hold down again and Save Recording. Now press options and go into Rockstar Editor again. Select new Project and add your clip to it. Hit Export at the bottom and give it a name. The trophy will unlock when the export completes.
There is a trophy for doing this 10 times. You can export the same project 10 times in a row: