1. Position yourself to face the chest sign (more or less), approximately 6-10 meters away. Now, I am saying meters but, of course, there are no units of measurements in this game. It's there just for you to get the general idea.
2. Hold the throw button and point the aim indicator crudely in the direction of the chest's sign. It's okay if you manage to aim even as far as 45 degrees or so.
3. Keep holding the throw button and instead of moving the aim indicator itself the trick is now to start moving your character instead. When the aim indicator 'sticks' on the chest sign, release the throw button.

If you don't manage to get it right at first, try to position yourself differently from the fist time, e.g. start aiming closer, or further, or in a larger/narrower angle from the sign. After some time you'll get the gist of it, I am sure.

Btw, sometimes, the hit procs even if the aim indicator doesn't truly stick and it is only aimed in sort of a general direction of the sign.