In one line: take an image of a 19th Century city map from one of the greatest sources of online maps, add your own airships, tall ships, machines, steam engines, mills, and other story specific brass and metal, put it all together, and print it out. Done.
Here's an example. Click for the full view:
Ready to begin?
First, say it with me now, "The Norman B. Leventhal Map Center at the Boston Public Library is my friend." It's sort of an incantation before we get going. (The incredible collection at the BPL contains over 200,000 maps and 5,000 atlases from the 1400's to present).
Step 1. Find a map that fits your background, whether urban, pastures, seacoast, etc. I've settled on a wonderful bird's eye view of 1860s Boston from B.B. Russell & Co. Here's where I usually start--with a search of "bird's eye views" of cities: http://maps.bpl.org/search_advanced/?mtid=42&dl_pp=1
Step 2. Draw or paint your own vehicles, machines, factories, places or things specific to your world. I drew balloons, airships, flying air-and-sea going ships. Draw these separately, in a paint app on your computer, or on paper and scan them in. (More about this below)
Step 3. Mod the map. For example, if you're going to have airships, your city is going to need mooring towers, beacon towers to direct flights, watch towers, guard towers (the shipping magnates who sunk all their cash in sailing ships don't like the air cargo ships eating into their markets), landing fields, floating docks, raised passenger platforms, all kinds of structures unique to an age of lighter-than air based travel.
The easiest way to modify an image is with an image editor that supports layering (e.g., Gimp, PSP, Photoshop, ArtRage--prices range from free to hundreds of dollars) and a tablet and pen (e.g., Wacom Bamboo at around $70 USD up into the thousands). I have a couple tablets, both Wacoms, an Intuos 3 that's my larger stay-at-home tablet and a new Intuos 4 that's my portable tablet. (For the image editing and painting apps, Google the app name + "tutorial"--also, make sure you have at least a decent handle on how these apps use layers. There'll be a bit of learning if you've never used Gimp or Photoshop).
I recommend you also use a pen and tablet to draw any of the vehicles or machines that are part of your setting. It's just easier than drawing on paper and scanning--at least for me. Do what's easy for you. If you have a scanner and you don't have a tablet, draw and scan. I actually drew the airships below on my iPhone using the Brushes app (very cool $5 app) and exported them.
Feel free to use these--resize them and modify them--for your own stuff, especially if you just want to play around, give it a practice try before investing time and potentially money into a map project. I've included them with transparent backgrounds to make it easy to drop into your map.Another way to do this, is to find the map you want, print out a large copy of it, and go to work directly on the paper with a pen, inks, paint. You have to be a little more careful (no undos), but this works just as well.
That's it! If you create something and feel like sharing, link to your maps in the comments. I'd love to see them.