I'll try to give an example of how I might previsualize a scene.
I'm out wandering around in the countryside with my Speed Graphic and film holders loaded with BW film, and I come upon a gnarly, old tree. About 15' from the tree is a boulder with some interesting grasses around it. I decide that I would like to create a composition that includes both. I begin to previsulaize how the final print is going to look.
First I have to come up with the composition. I can randomly choose a focal length, and wander around looking through the viewfinder until the subjects fit within the composition. Possibly they won't do it like I want, so I randomly pick another focal length and try again. Or I can use my knowledge of how distance to subject affects perspective, and how focal length affects the angle of view and the size relationship of subjects at different distances from the camera, and make an educated guess as to what focal length I want, and where I should stand. I still have to look through the viewfinder to make sure I'm right, and make corrections if I'm wrong. Both methods can be previsualization, the second method just takes less time (hopefully). In this case I'll choose something fairly wide angle, because I want to put the boulder in the foreground, and I still want all of the tree showing in the background.
Now I have to consider depth of field. I usually follow the traditional school of thought with landscapes that I want as much as possible in focus, so I know to achieve that I'll need a small aperture (my Super Speed Graphic has some front movements, so I can also use this to affect DOF). I'm not sure that my eye sees it this way, but I can imagine what this will look like as a print. My eye sees everything it's directly looking at as pretty sharp. I know to achieve maximum sharpness in the print I will need to use a tripod, and cable release ( and mirror lock-up if I were using a SLR ). Because I'm using the smallest aperture, I know my shutter speed is going to be slow, so if I don't want the movement of the grasses in the wind to show in the print, I'll need to wait for a break in the breeze.
Next I study the tones. There are some nice clouds in the sky, and when I previsualize the print I see a medium gray sky, with bright white clouds. I know that if I shoot this scene straight, I'll get a very, very light sky, and the clouds will hardly show, because the sky is very bright, and BW film tends to overexpose blue. To deal with this I could use a red filter. That would make the blue sky very dark, but it would also darken the green leaves. An orange filter would lessen these effects, or I could use a yellow-green filter, which should help darken the sky as well as lightening the green foliage somewhat. Maybe I'll take a shot with both the orange filter and a shot with the yellow-green filter, because my previsualization isn't perfect, and later I'll want both options. Because I'm using a hand held meter I need to consider the effects of the filters on my exposure. I'll need to overexpose a stop or so to compensate for the light blocked by the filter.
Comparing the detail in the bright clouds with the detail of the tree bark in shadow my meter says they are 7 stops apart. I know that my printing paper can only handle 5 stops and still get the full detail. To compress the tonal range into something that will print the way I want it I need to overexpose by 2 stops, and later I will adjust my development time to under-develop by 2 stops (how much time this is depends on many things, so each photog needs to do personal testing).
Now I can take the shot, and get reasonably close to what I imagined (previsualized). This may read as complicated and time consuming, but because I've been doing it this way for years, it's fairly intuitive these days, and it all happens in less time than it took you to read this. I'll spend most of my time waiting for the grasses to sit still.
Previsualization is just looking ahead, and thinking about the photograph you want to take. Choosing color film because you know you want a color print is previsualization. You can try and previsualize as much as possible about the photograph, or just certain aspects. It's not really as complicated as it sounds.