First, thank you for your answer. And I didn't even knew about the layer.

Right now, I'm constantly switching between GIMP and Photoshop. But I have the feeling that I do it for feature availaible in both software. Any suggestion for tutorial? (I guess there might be a lot on this site)

I'm bad at drawing, but make up this shortcoming by being average at math. This allow me to use angle and x,y,z coordinate to compensate. I don't know if one of this software has better rulers and x.y.z positioning than the other?

And I'm curious as to the best starting resolution/size for doing a work. I am alway at a loss as to what to input for this blank sheet.