Let me give you an example. While doing research for a book about the Civil War (Photo by Brady: A Picture of the Civil War), I ran across a fascinating battlefield tidbit (of course, I ran across dozens, but I'm just going to use one right now.) The tidbit was the information that at the end of a battle, sodiers' faces would be black around the mouth from gunpowder, because they had to bite off the ends of their paper cartridges, and in the frenzy of battle the biting and tearing got a little messy with gunpowder splashing and spilling.
Okay, what can we extrapolate from that? I like to consider all the senses when I have to flesh out details. "Faces black with gunpowder" is a vivid visual detail, but it also suggests other sensations: the gritty feel of gunpowder between the teeth, not to mention the taste of it in the back of the throat for hours at a time. (Full disclosure: I have not tasted gunpowder so I don't know how to describe it.) It suggests the sting of gunpowder in the eyes or up the nose; I can imagine spitting black spit and blowing black mucous into a hanky. Does everything smell of gunpowder when it coats the inside of your nose? Do you spend the first hour after battle spitting and rinsing out your mouth, provided you can get water? The gunpowder must also be in the ears, the hair, down the shirt collar -- everywhere. If you've been sweating no doubt you are smeared with black sweat, and the creases of your skin will be etched with black powder. Chances are that a right-handed soldier will have more powder on the right side of his face and head, and vice versa for a lefty.
Thus with one sensory detail, I can extrapolate a whole panoply of contingent information. It takes some practice, but anyone can do it. I find many kids are unaccustomed to making logical inferences, so when I demonstrate this process to young readers it looks a little like a magic trick, or like I'm just "making stuff up." But trust me, it's not really pulling a rabbit out of a hat -- you just have to look carefully inside the hat and see what's in there.