So, you want to vector eh? It's basically a simple method of getting a scan or a drawing and overlapping it with higher resolution vector and vexel colors.
I usually take an average sized scan let's say around 600x450 and I blow it up to 3000x3000. You need to keep the quality even if you're using it only for a sig or something.
You have to be meticulous as hell and have a lot of time on your hands. If you're on Photoshop, you'll be using AT LEAST
60 layers. Just group em' put em in folders and be organized. Lets start shall we?
First thing's first though. you need to learn how to make the line art.
Here's the tutorial I used when I was learning myself it's really simple but takes a long long time.
I'm using Photoshop, it was easiest with a simple learning curve. Once you get that down, you better upgrade to Illustrator, smarter but a little more complexed.
Spoiler for size. The line art is key for a good quality vector, super practice with the pen tool is necessary. If you got that mastered you'll never use a polygonal lasso tool ever again.
Now, after you have the foundation of your scan/art in place it's time to color it. Here's the basic color guide from the same guy. It's all about the color palette, make a good use of the hue of your character, the shadows, the type of shading. Take everything accountable.
Lastly is adding the details to the character, the shades, the eyes, the strands of hair.
It takes a lot of work and a lot of practice but in the end it really is fun to do. I'm able to line art with a tablet now but I still love using the pen tool.
I wanna give credit to tsukishoujo
for making an awesome tutorial, a very very talented artist.