So, I’d like to make my particles follow a trail to make the shape of different things. The most simple example would be a letter. Say I wanted my particles to emit from birth to death in the shape of the letter ‘a’ - how would I do this? I should add I’m really new to this, so pointing me in the direction of the right concepts and learning material would probably be more awesome than telling me how to do it, but either way please help! 
PS (In the context of GPU sprite emitter would be best for my needs)
Maybe with collisions? Like if you were doing it for video? Emit the particles inside the object with the object acing as a collision surface. They would bunch up inside the shape that way. That’s a very interesting idea, I just saw how Pixar made all the characters in ‘Inside Out’ that way. They aren’t models just huge particle systems bunched together so tightly that they look solid but when they react you can see the particles moving around. This could make for an interesting character if you could get it to work.
I get you, although I’m not sure that will leave me with as much control as I need. What I’m actually trying to do is draw musical scores with particles, but the layout of the scores has to be able to update when certain events are triggered. If I used some kind of collision objects, I’d have to be able to update their location on the screen relative to all other collision objects that make up the score. I need to create shapes etc for every visual layer of musical score, which is something I imagined doing by defining the direction the particles move in, and the path they follow in 3D space, as if they were actually writing the shape. To create the full score from all these shapes, I’d attach the emitters to one object, and update their positions relative to that objects position. I haven’t really thought this far ahead until now to be honest. I only did my first particle tutorials yesterday!