Of course the script comes first (and that comes after the research and plot workings-out), but while I'm writing the script, I'll rough out the page simultaneously. As Nazaleod is in 4-page episodes, I do these roughs on A4, folded in half to give me four A5 pages.
Then I pencil the page using a Rotring mechanical pencil with a 0.5mm H lead and working on Goldline A3 220gsm bristol board. The page is inked with a dip pen (Hunt 107 nib) and india ink - as you can see from the accompanying image, I scanned this stage for some reason, with pencils still underneath, which I don't usually do. After the pencils have been erased away, the inks are scanned in to Photoshop as a 600dpi bitmap, and it's here I'll do any little corrections and add any 'white ink' (eg. the rain). Finally, the bmp is converted to colour, the black line is lifted to its own layer and colour is applied underneath, before being transferred to an A4 (actual size) master and lettered (not shown).