It’s simple to trim the jib when the telltales show a problem. Move the sail in the direction of the fluttering telltales. If the fluttering telltales are on the inside of the sail, as shown in this photo, then pull the jib in tighter until they are streaming back horizontally.
If the fluttering telltales are on the outside of the sail, then let the jib out until they are streaming back horizontally.
Depending on the sunlight on the sail, it may be difficult to see the telltales on both sides of the sail at the same time. You may have to change your angle of vision to catch their shadows. With a little effort you can usually see the telltales on both sides.