The process of illustrating with a grid in Adobe Illustrator
First i selected a photograph so i can use as a background for my illustration. It's important to have a clear shape and different colors.
I begin by locking the layer with the image and making a new layer on top. On this new layer i begin making triangles with the pen tool, making smaller triangles in the areas that need more detail (Like the eyes).
As your grid get's bigger, you can begin to see a 3d shape, and it's important that this shape resembles the original shape you're trying to illustrate.
Now our grid is finished. This is when i begin to look for mistakes. The most common mistake is a triangle that is not properly alligned with the grid, creating "holes" in the grid.
The coloring is pretty easy. You just select a triangle and with the eyedropper tool select a color from the original image. Many times you won't be able to select a triangle, because it will be underneath other triangles. Don't mind that and continue. You will be able to select those triangles one by one in the layers window.
In the process of selecting the color for each triangle, it is important to see the colors of the triangles surrounding the selected one, so the color is not so similar. But also you have to select a color that represents the area you're coloring.
Now the illustration is almost done! Now you can remove the background to see clearly if any white "holes" appear in your grid.
I usually add something else just to make the illustration not so similar to the original image. More triangles usually do the trick!
And that's it! now you can put your illustrated vector on top of the original one, or just put it in a solid background.
Camilo Güell