How to Make a Black Mirror for Scrying
This is probably the most simplistic thing to make, but also super effective.
The first thing you want to do is get a simple small mirror. You can go to the dollar store or pound mart and pick up any small mirror you want for a couple dollars or you can order something simple on Amazon, but I wouldn't spend too much money on this initially.
The only detail I would say about the mirror is I would try to avoid really ornate frames around the mirror itself (and if you can avoid purchasing one with a frame at all) as that can be visually distracting when you are trying to do this exercise.
The next thing that you're going to want to pick up (if you haven't got it already) is some black paint. As black as you can get it is preferable but any shade of black will do.
You are then going to paint over the entirety of the mirror you have. All the "mirror" portion should be covered and when dry should be dark. You may need to do a couple layers of paint in order to successfully complete this step.
And...that's it.
Once it's done you can put a finishing coat to waterproof it or help prevent it from chipping if you want ( I personally don't) and then you are ready to use it for all your scrying needs!