Tools :

MS Paint or any other pixel art program. MS Paint is more recommended if you want to make an Undertale sprite for an Undertale fangame because the sprites in the game are made in MS Paint.

Tips :

Any other pixel art program :

Use the line tool and not drawing a line your self so it can be straight, depending on the sprite your making, if you are going to make a Sans, you draw the line yourself, that`s why there is line size inconsistencies on Sans` sprite.

Use the available shape tools.

On MS Paint only :

Do not use the brush tool, use the pencil tool.

Use the shape tools.

That is all.