There is method to the madness. First, I usually draw a freehand sketch, from which I take the main lines or shapes and emphasise them with pencil. I then start filling them in with fineliner, thickness 0.5 or 0.7.
Then I’d add more detail with pencil and if happy with the result, draw it in with fineliner of the thickness 0.2, 0.1 or 0.05.
Colour comes at the end. I normally work from left to right and top to bottom.
Cadogan Square is an A3 I drew within 9 hours.