Everyone who has ever wanted to draw fantasy creatures, has drawn themselves a dragon. Those who fancied themselves something with a pencil, or found themselves very, very bored, have continued on to draw scales. Personally, it was probably one of the first real detail techniques I ever learned.
There are two major methodologies in creating those scales. The first is the long way, render every scale. The second is the short way, figure out the basic texture formed by the scales and then scatter a few hints of that texture about. If you aren't really going to be rendering anything, it's always preferable to just take the second method. You first need a bit of practice drawing scales to figure out the texture, but honestly once you about know the forms you're trying to get across it's pretty simple.
Drawing a full set of scales though, well you can do it pretty easily actually. The difference is, you block out where the scales should be and then go to town. You make a single mark, roughly circular but not truly needing to be. After that you make a second circle touching it, then you create an arc traversing a short angle between the circles. The real key, is just to make one scale at a time. If you try and produce entire rows or sets at once, it'll come out bland and probably too large. One small arc, one small scale at a time and the whole pattern just eventually works out. Personality is immediately apparent as each arc vies for it's place in it's microcosm, sitting next to those that happened to be drawn slightly larger or slightly smaller. Most with their entire shape predetermined by the minor variations in the scales around them, but seen from too far away to appreciate. At the same time, they are seen too close to understand how that one circle so far back in line had it's own secret hand in it's final shape.
An entire complex and deeply detailed structure, made from simple rules and some minor variation. No thought or design into how they would all fit, no thought put into the last scale in line and not made thinking of the next. Just a simple rule and a margin for error.