Step 5: The drawing surface
You can use a wall if you have some motor mounting brackets, but I was always terrified about it going wrong and scrawling marker pen over my wall. My landlady would be unimpressed, I feel.
Using a board means you can tilt it slightly too just by leaning it against the wall, and that's a good thing because the weight of the gondola presses the pen to the page. When the surface is perfectly vertical, it's hard to get any pressure against the page - the lines tend to come out a pretty woolly.
I went down the local DIY shed and scavenged in the offcuts bin for the biggest bit of chipboard I could fit in my little car, but I've also had good success with building a machine based on the biggest IKEA Ribba picture frame. This has the added feature that you can use it as a picture frame afterwards, amazingly. A whiteboard is a good alternative too, because you can test quickly, but any kind of flat surface will do. My first one was only big enough for A3, and worked fine, so don't feel it has to be massive.