I have been asked "Do you have a drone"? or "do you go up in a helicopter"? I wish i did. ;]
Below is a brief explanation of the process i go through, in essence i do months of research before i even put, pencil to paper.

My large drawings take up to seven months to complete.
They start with a clear idea of what i want to achieve for the final piece of work,i then do as much research as possible of all the areas i have in mind. Photos from every possible angle and close up detailed images.
i walk around these streets taking photos and sketching.
London is constantly changing so i try to capture as much as i can before it changes.

I then sketch out my plan/idea on a large, best quality thick watercolour paper, which usually takes a whole day.
This is the crucial part , as i then have to decide if i think the composition works and if its worth spending the next six months of more fine tuning and continued research on the areas and adding bits of History, personal stories and humour, hidden in the drawings.