Once you have uploaded a video to your theater, go in to theater, pick that film and watch it.
You will see a toolbar on the screen, on the far left is a picture of a camera.
Using your Left Thumbstick, go over to the camera and press it.
You then will have taken a screenshot and it will give you options of autoname/save or naming yourself or discarding.
I picked autoname/save just so that I'd get the achievement.
Should pop up straight after!