You have to hang as many strings as you want ornaments. Few ornaments means a less defined tree. You can hang them from tacks in the ceiling, but its extremely labor intensive and can mess up the ceiling if you use too many.
i would make make a grid out of wood or small dowels to attach it to, so you only then attach that to the ceiling. i do think it would require frequent maintenance to prevent the tangled mess though.
I've begun to make this for this year; it's something different.