I doodled these very roughly with red biro white I experimented with shape and size. I went over them with my black artist's marker, then shaded them with my grey pro marker. I made two main designs, on the left and right hand sides, and on the bottom of the page is each of the two as seen from the side. I think I much prefer the one on the right: it looks more interesting both from the front and the side, and I think the proportions and perspective are a bit easier to draw than the left-hand one.
The next step will be designing the interior, then colouring and texturing!
