To the left is a preliminary sketch of Chessa, the dragon who appears in Fiction in Action: Spellbound, Adam Gray's book due out for this autumn. In the case of this book, the text is written first, then drawings are commissioned from the description. But above all the drawing has to work as a drawing. It may be the drawing has to move away from the textual description's non-essential elements in order to work in terms of angles, in terms of look or feeling, in terms of color. As it turns out this preliminary sketch differs from the text in a number of ways. The decision now is how to proceed. Do you stay with the sketch and change the text or vice-versa or do something in-between?
In the case of a drawing like this, you're working forward from the text and then potentially working backwards to tweak the text. But there are many cases where it's easier to begin by working backwards. Maps are an example that come immediately to mind. Decide the elements/points you need included and draw the map first. Then make your exercise or text based on the map. You can then do some forward tweaking, backward tweaking, until you've got it as you want. But if you start with the text, you can end up with a situation where it's almost impossible to depict a map faithful to the description without substantial change in the text (and as a corollary to this, whatever you do, don't ever, ever go to recording before you're sure that the map needed for the exercise can indeed be drawn!)