Black Soul Gems, Im still confused Options

[Recros]

Ok. so i read the last post where it talks about how to make them, but im still confused.

So first you get a grand soul gem(which i have) empty or filled. Then you put it on an alter at night. So what is this alter that you are supposed to put it on?? is it the alter in the arcane university where you make you spells or enchantments??? or is it some other alter that you are talking about???