Well, in the abstract, you could begin by literally experimenting. The "sound" you have created, whether it be a single instrument or a few, still has sonic characteristics. As a first step, try adding something that you perceive to have a lot of contrast to those characteristics. Such as if the sound is very slick or smooth, add a layer that is grainy or gritty. If your original sound is very continuous or sustained, add a sound that is choppy. You can also add layers of very similar sounds and create a "build up". In every case let your ear be your guide. Very quickly you will find that certain combinations are more interesting to you than others, and from there some answers should emerge as to the next steps to take.
A great reference point is music whose sonic qualities you admire. Even a small section of a song can point at something you have been looking for. I dont write music ANYthing like Imagine Dragons, but the song Thunder has what sounds like an instrument solo with is actually Reynolds close mic-ing himself with a filter and in the CD version the voice is removed leaving only the filtered version. (Live it seems the voice is retained in the mix) Point being: that sound is something that sent me looking for a way to replicate it.
If you hear something interesting, pursue it. If something is missing, you don't so much hunt for the sound as try to grow it. Well, maybe its a bit of both.
