This is a step-by-step tutorial on drawing scales. For learning purposes, I've drawn them as though they were on a flat surface, but keep in mind that on animals, there are none. I'll explain how to modify your drawing to look as though they were on a curved surface at the end.
1) Draw the guide lines that would run along the outer tip or edge of the scales. Keep in mind how long you want your scales to be. You should draw one of these lines every half-length.
2) After that, draw lines going diagonally across the horizontal ones. Each "diamond" formed by these lines is one scale.
3) The next step is to draw the scales themselves. They don't have to be the exact shape of the diamonds, just make sure that the edges of every scale you draw go outside or inside the diamonds the same way. In these drawings, I'm making each individual scale in the shape of a semicircle, but you can use whatever shape you wish.
4) After you've done this, you can erase your guidelines.
5) Now, if you wish, you can color them and/or add in details, such as a ridge running up the center.
By now, you should have drawn a nice set of scales. But they still look as though they were on a flat surface. To make them look curved, modify step number one by making the lines curved instead of straight, with the peak at the part of the animal that would stick out the most. Make the lines you drew in step 2 curved as well, with the peaks leaning towards the part of the animal that would stick out the least. Remember that the further the change in depth is, the sharper the curves are.
Good animals to look at for comparison would be snakes and pangolins.
Once you've grown confident with your abilities, you can skip over the first step completely.
I hope this "how to" helped you out. Feedback is always appreciated, and if you have any tips, websites, book titles, or the like to contribute, please do so.
Good luck with your drawing!