I'm exclusively a side sleeper. You use a U pillow, which the X5A rests between (something akin to the neck pillow you typically see used for airport travel; there are a lot of variations to test to find whatever works best for a given person in terms of comfort). There are also a lot of variations of cut-out pillow options for headphones/earbuds and ears (people with sensitive ears), for side sleepers.

So: X5A hard plastic ear segments between U shaped pillow (the U shape supports your skull, the hard plastic goes between the U) -> then ideally something very soft under the U shaped pillow which buffers between the hard plastic of the X5A and your mattress, so the plastic never fully presses against anything firm (if the plastic of the X5A meets something firm, then your skull is meeting something firm). Very soft memory foam can work great for that, or really anything super soft.