Lineart

Since I had to backtrack on this picture, I started out with lining Sans.

Lineart Part 2

I then did the lineart on all of his skulls but the one in front is the one shown here.

Coloring

I started off coloring the skulls first and adding their glow. I decided to do more of a soft shade rather than the cel-shade I'm normally known for to help Sans stand out more. I also blurred the background skulls in order to give more perspective to the picture.

Coloring Part 2

I then colored San, again I color on the same layer so I apologise this isn't separated more. I like using high contrast shade on my cel-shades.

Coloring Part 3

I made the background black and then started on the heart which I wanted to make glow but also be broken since this is the genocide run Sans.

Finished, Complete

Add the extra glows, tidy up, and put my watermark and it's done.