Sprite Sheet Animation or 3D on 2D.

Well, I am new to the sprite sheet business...

I recently learned Unity and am fully interested in making 2D games...

(Later focusing on making 3D entities look 2D in Orthographic view for 2D games...)

Well, here's the thing... I want to learn sprite sheet animation without the use of any add-ons...

And, I found that there are tutorials only about using Sprite sheets along with 2dtoolkits, orthello, sprites manager 2, etc.

Would anybody help me on how to do Sprite animation without any toolkit involved with hard work only??

Only a learning path is enough for me!!

*And , My focus is only on Sprite sheet animation in Unity...Just to be clear!!

Or,

Is it better to work with 3d models on orthographic view in Unity to make it more realistic and 2D for 2D games.*

P.S. It's my first time on Unity forum so please go easy on me.