Most of people usually use Screen Tone for background these days.
But we introduce the point of drawing background with a pen here.
Perspective
When you draw tactile things,
basically you should draw a thing in the foreground bigger and a thing in the back smaller.
But when you draw a wall or a floor, it is not like that.
This is called one-point perspective projection.
With this way, you can draw a ceiling and a depth correctly.
A heavy line shows your eye line.
This picture is drawn in two-point perspective.
You can use this way when you want to draw two walls.
A heavy line is your eye line.
It is useful to draw a turn or a suitably-angled building.
Composition.
You can draw impossible composition on cartoon.
Why don’t you try to draw your original composition?