These instructions are for Photoshop, btw. I tried to keep it as simple as possible so I present: Subway Art in 10 steps or less! (It's really 10 steps so you can see how the whole thing came together.)
First, create a new canvas by going to "File", "New", and then creating the actual canvas. I started out with an 8x11 72ppi blank canvas then used the paint bucket tool for the background color.
Using different size fonts is key. You want to make it look random without it actually being random. Plan it out in your head & make changes as necessary.