Step 1: Get a pane of glass or picture frame with pane of glass

Step 2: Paint the back of the glass with glossy black paint (preferably in multiple coats), letting it dry completely before the next step

Step 3: After the glass is a black mirror place it back into the picture frame, or carefully wrap the pane of glass' edges / make a frame for it

Step 4: Perform the empowering ritual here: https://www.satanisgod.org/www.angelfire.com/empire/serpentis666/Black_Mirror.html

I personally have both an obsidian mirror and a glass black mirror, both work fine. It is in how you use the mirror that will make the difference, as it is a medium.

Also of note; When not in use the mirror is to be wrapped in silk or satin to keep energy from escaping. (I personally use a satin pillowcase to store my mirror) The mirror will need recharged every so often, and to do this you breathe lunar energy into the mirror as in the empowering ritual.

Read the whole JoS website. You'd've known how to do this if you had read the site in whole :/
Praise the sun!