Ok, this is my first post, but I wanted to make a contribution since a lot of the tutorials here really helped me start making maps. If you are looking for a decent looking city map, but nothing fancy, this is for you:

First, use RPG Citymap Generator to create a city you like (I have yet to try anything massive),
After you get the city you like, select the "Export Selection as Images button"
In this menu make sure the "Divide Section" drop-box is set to 1x1,
Next, select the "labeling" button, deselect everything, you will probably want to label in gimp anyways,
Now click "Colors", from here give your trees a good green color, and set the Park color to any distinctive color, make sure your walls is something you would like also (If you have walls),
Next, set the pixel size. I recommend 2000 for smaller towns, and 3200 for larger towns. Make sure the "Buildings: Only Outlines checkbox is deselected.

Now, once your image is done being made, open Gimp and close RPG Citymap Generator. From here I recommend following Rob A> 's "Creating Old Paper/Parchment in Gimp" tutorial for some old paper (Or another similar tutorial if you choose).

Below the "Fuzz" Layer in my image (Or on an appropriate layer on your paper) I created a new layer called "content".
Next go to File > Open as Layers, find your city image that was created by RPG Citymap Generator, and click open.

Now your city map should be on the paper, probably looks bad though, so lets do a couple of things to it.
First, set the layer mode to something that makes it look better (I find that Burn works best),
Now, scale your city map so it fits appropriately (Anything hanging off the paper should not be seen),
Now, select the "Select by Color" tool and pick your park color, go ahead and fill it in.

At this point the rest should be simply editing the colors a bit, adding labels, etc. so I will cut this off. If it gets a lot of feedback I will try to put together a more thorough pdf tutorial. Here is a picture of one of my maps, I haven't done too much with changing building patterns or anything, but it looks pretty good. Im working on a much better one that is an above ground city (Has parks and whatnot)

Click image for larger version. 

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