The following should apply to both open- and closed-eye visualization. I do some of both but prefer closed-eye because there's less going on in my visual field to distract me. Note that I do still often struggle to get things going, but the stuff in this guide generally does work and at least get me something. It might not be really vivid, but it's way better than darkness. Also, this guide is a bit TL;DR, sorry.
To clarify, this is about "seeing" things in your mind's eye / imagination. You will not actually see things visually until imposition. However, you should get something that feels a bit like seeing. Best I can really describe it is like it's on a separate plane from your real vision. It's like what you "see" when you recall a really vivid memory -- you imagine it in full color, and with good detail, but your real vision always overrides it, especially if anything is there to distract you. This is why closed-eye/blindfold/perfectly dark room so often work well, and why it helps to get rid of visual blobs and noise as much as possible. When you impose, you're allowing this same imagined picture to override parts of your real vision.
First off, the biggest problem I see is people trying to make stuff appear on the backs of their eyelids. If you're doing this, don't. Focus instead on your mind's eye, where you imagine things, like when you think visually, read vivid descriptions, etc. Learning to shift your focus from your vision to your mind's eye can be difficult, and certainly was for me.
The very first thing to try is relax. Being a little to a lot relaxed often makes it easier to see yourself somewhere. Thinking about being somewhere happy -- not necessarily seeing anything -- may help you relax as well. Try listening to soft music, hard rock, binaural beats, Fede tones, whatever helps you. If you can, get to a trance state, but that's something for another guide. Just let yourself space out. ...Or don't relax. Sometimes you might just randomly get amazing visualization in a full-waking session.
If closed-eye doesn't work, try open-eye, which is about the same as daydreaming. Something in between is to have your eyes open but wear a blindfold or sit in a room with zero
Do you get any activity on the backs of your eyelids, like excessive noise and "plasma blobs"? I find these are quite distracting and make it way harder to focus on my mind's eye vision instead of the blobs. Usually relaxing my eyes and not squinching them shut helps with this, as the muscles around the eyes press them when contracted, causing the blobs. If you can't live with the blobs and can't get rid of them, play with different ambient light levels in the room. Pitch black is not always best. Medium-dim indirect light can produce a flat orange across your whole field of vision, with very little discernible noise or blobs.
What is it like when you try to imagine something, either open or closed eye? Can you at least get a sense of what is where in your imagination, but just can't "see" it? If so, you might just need something to draw your attention to your mind's eye. If you don't even get an idea of what's where or how it's supposed to look, pay extra attention to describing what you want to see with words.
Imagine something vividly colorful, with lots of color, and that moves at least some. Lots of contrast is good too. Places you're really familiar with can work well too. I'll sometimes use my childhood home, or a beach with really vibrant, exaggerated colors. Try to describe what you want to imagine with words, and include as many senses as possible, especially smell and sound. Try to imagine how light and shadow work, and try moving the light source around to see it change. If you're imagining an indoor place, imagine turning on the lights. If you're outdoors, be sure to include the sun or a full moon. Also consider imagining a small flashlight in your hand. You can use this to illuminate an object you're examining from any angle, and move the light and shadow around it freely. (See below for a detailed example.)
Occasionally I've had success with imagining all black, then all bright white, alternating back and forth -- a strobe of sorts. The idea is it's sudden changes and about as much contrast as you can have. Still, realistic scenes typically work best because they involve more senses.
Once you catch a glimpse of something, you need to hold onto it. If what you're seeing is interesting enough to you, this may be easy. If what you see vanishes instantly, it may be because of a thought like "cool, I'm seeing something!", which makes you snap your focus back to your eyelids to see it. Instead, try to remain spaced out when something comes. Do not focus on seeing things; focus on the things you're trying to see. When you start to see them, don't do anything different or think anything in particular of it. Just keep focusing on what you're visualizing and continue normally.
If you see something for a while, and then it fades more slowly, leaving you staring at your eyelids again, you've lost focus or drifted out of whatever favorable (relaxed, zoned out) state you were in. Speaking aloud can do this, as can excess movement, at least for me. As for focus, that's another thing that takes practice, although I've heard good things about bananas and an energy drink called Neuro Sonic. If you absolutely cannot focus, imagine a place where there are plenty of different objects, and let your attention dart from one to another. You're still focusing on things in the same mental location you're trying to see, so it should work.
Example way to handle a beach scene.
If you don't like beaches, substitute any place that you feel really happy in. This may or may not be your wonderland, though that should be a happy place as well. Merely reading this example may well make you see something.
(Change the order to put whatever sense comes most easily to you first.)
Don't concentrate too hard on making something appear, or it won't. Narrate these details to yourself, or later your tulpa, as you attempt to imagine them. Even if you don't see anything, continue. At some point, you will probably catch a glimpse of something. If something does appear, don't focus on it or go "wow, I'm getting something", or it will vanish as you immediately snap your focus back to your eyelids.
- Sight: Start with the sky, which is clear and a deep azure that is deepest straight above and lighter toward the horizon. The sun is about halfway down the sky, on your right. Let your focus drift downward from the sky and see the ocean water, slightly rippling and with small waves. Now see the rich, warm golden sand, closer to brown where the water has touched it, stretching from the water's edge up to where you stand. See the dips in the dry sand where people have walked. Notice how some random grains of sand glimmer in the sunlight. Point your finger at the sun, and make it follow where you point. See the effect on lighting and shadows, including on each dip in the sand. Also look down and see your own body; this helps place you in the scene and establish that you're there, and not just a floating view.
- Smell: That unique salty smell you get at a beach... Or that stinkier low-tide smell if you like smelling icky things.
- Sound: Hear each wave as it wooshes in, crashes and then recedes. If there's much breeze, hear how it sounds blowing into or past your ears. Add some seagull or other water bird noises if you like.
- Touch: Feel the soft sand under your feet and feel how it shifts as you walk, your feet sinking into it slightly. Reach down and pick up some sand. Feel how it's warm from the sun, feels gritty and has the occasional really small pebble in it. Feel it flow out of your hand between your fingers, leaving a slight coating of sand stuck to your skin. Brush it off, feeling how it rubs on your skin a little. Walk down to the water, feeling how the wet sand is cool and hard under your feet. Walk slightly into the water and feel how the cool water flows by. Pay attention to how the water washes away the sand directly under your feet as a wave recedes. Reach down and scoop up some soggy sand. Notice how gloppy it feels for the moment before enough water runs out of it that you're left with a wet crumbly lump. Try to shape the lump a little, then drop it and feel how it's left your hand damp, with small amounts of wet sand sticking to it.
- Temperature: Imagine that there's not much wind blowing. Feel the warmth of the sunlight on your skin wherever it hits you. Now a cool sea breeze picks up, coming in from the ocean. Feel it on your body, especially on bare skin. It reduced but doesn't eliminate the warming effect of the sun. If you're wearing loose clothes, see and feel how they flap about in the breeze.