You know how when you close your eyes it's not 100% black? There's that "snow" type thing like you see when a t.v. isn't getting any reception, and you may see some faint colors and shapes, thought it's mostly random crap that never really amounts to anything. Try looking at a picture first. I used your avatar. Then, close your eyes and imagine the image being formed out of those abstract shapes and snow-like effect. For example I first pictured the general outline of the character on your avatar, followed by the weapons outline, wing outline, and then filled in the colors. Granted you don't actually see the picture, but you can still imagine it. The image becomes the clearer the better you get.

Once you get decent at that you can start creating images out of nothing. This is, of course, only limited by your imagination and certain people are innately better at this than others but there is no real limit. Also, daydreaming can help a lot with this.

This is at least how I do it and, of course, it's not going to work 100% for everyone but it's a start.