To kickstart, I copy tons of photos of that animal*. Now, we learned in art school "copying photos' a big no-no." Yes, I agree, 100%. If you copy photos and have it as your final art, it risks (A) being static and dull, and (B) OH GOD all the copyright infringements**!!
But this part of the exercise is to loosen up and study your subject matter. They are foundations that will come in very handy in the future.
Most of the time, I just do it on scrap papers. These are quick exercises, meant to be seen by no one. But doing this repeatedly does magic – I now know how to do a giraffe! It's in the memory of my creative muscles!
The next step is to translate the giraffe to how I want to interpret it.
And bringing it to a final piece.
Ideally, if time and money allow, I would go to a zoo as well to observe the real animal. But doing such an exercise prior to sketching in real life brings great benefits: I can now fully concentrate on the gesture and motion of the moving animal, not just how it looks.
It's fun, and it's helpful!
* Either local library books or online images will do. I like to go on EOL for references.
** Fellow KidLitArtist Jen Betton has an excellent blog-post series on copyrights: