If this is something you’d like to try, or if you’ve tried in the past and couldn’t get it to work, you probably just need to practice!
A clear depth seems to work the best. Anything clear is good, from a glass bottle filled with water, to a clear glass crystal ball, to a pricier quartz ball. You want it as clear as possible, but some occlusions are okay if you’re using quartz. The idea isn’t to sit in daylight looking at occlusions, though. You want dim light, anywhere from a shadowy room on a heavily overcast day to moonlight. Arrange a dark cloth around the crystal to shut out the reflections and such. You should see just a round, dim, greyish glow from the bit of ambient light that it grabs. Look into that and when your mind starts to wander, as it unavoidably will, bring it back to the crystal. This is what trains your mind, in time you will get results pretty quickly. It’s like meditation, except you’re focused on an outside object.
Practice for about 15-30 minutes at a time. Three or four nights a week is adequate, but more is better. You should see clouds and pinpoints of light in it pretty quickly. Persist. In time you will see the sphere disappear and give way to a dense blackness. Again, persist – results in most cases are slow and gradual. It could take a few weeks or the best part of a year, but eventually you’ll see pictures.
If you prefer using a black mirror, that will work, too, but whatever you decide on, stick with it until you get results!