1. Pick a video game. Preferably something with a lot of consistent imagery/gameplay. A racing game running the same map would be a great example.

2. Play this video game from when you wake up to when you go to bed, with minimal time for breaks or distractions.

3. After hours of having these images burned into your retinas, randomly try closing your eyes for just a moment or two and rest your brain every once in a while.. When I'm playing video games intensely and then I shut my eyes, sometimes it's like I never even shut them in the first place - all the images are still mostly there and still mostly behaving as I've been watching them behave (e.g. I'm involuntarily visualizing the walls rushing by as I make turns in the race, etc. My eyes feel funny and then I realize they are actually closed! holy crap! etc...).

I've found first-person games like FPS or racing games to be the most intense and reliable in producing this effect. You might also get better results at different times of the day (e.g. alone in a quiet room in the middle of the night). But it works with any game, or really just in general I can close my eyes and have the scene flashing in my mind.

Here's another thing to try:

Walk casually around your house or another familiar area. With your eyes closed. Only once every few seconds (or when you think you really need to) - as quickly as you can, blink your eyes open and instantly shut again. Try to retain as much information as possible for the next few seconds of your blind walk so you don't run into things or step on things. You will be amazed at how normally you can perform with scant visual information.

er, also note that trying to visualize something from nothing can be extremely hard. For example I cannot look at someone's face and then imagine them in all kinds of new facial expressions that I've never actually seen on them before. If you try to change something you're not really equipped to visualize, it will just seem like an amorphous blob or an abstract symbolic designation rather than striking visual imagery. If this happens with something like geometry, that probably means you just need to spend more time trying until you get it, but don't be too surprised if you're trying to visualize this massively detailed scene of real-life visualizations, and things just aren't as vivid as you like. Visualizing geometry and relatively abstract scenes is way, way less demanding than trying to manipulate the full visual resolution of real-life images in your mind.