Trying to learn how to use thumbnail drawing

In these moments of my life I have found a big problem, although my 3D modeling is improving more and more, it is vital for me to reinforce the 2d and when searching for tutorials I found this interesting technique (thumbnail sketching); that is why I want to show my first attempt with him and I will be sharing new attempts little by little so that those who can do me the favor of giving me an opinion and / or advice to improve me do that great favor.

Thank you in advance for your support.