Creating a campaign map in GIMP

This tutorial assumes you are using GIMP which is a free image manipulation tool (poor mans photoshop essentially).
If you have any feedback feel free to let me know at admin@jsigvard.com or on reddit at /u/thewardvg.
Below is my own campaign map I created with this method as an example of what your map could look like at the end of it.

Basic Outline

1. Create a new image. The resolution is up to you really. Personally I use 3000x2000. Its important to not go to small as you want to be able to label smaller settlements etc. without having the text be illegible.
2. Fill the entire image with black and use whatever way you prefer to draw a rough outline of white. Personally I use the "Free Select Tool".

3. Apply a gaussian blur. Filters -> Blur -> Gaussian Blur -> 200.0 Blur Radius. The radius can be changed up and down as you see fit, with a higher radius resulting in deeper "divets" in the coastline. You probably want lower if your are using a smaller resolution image.

4. Create a new layer and fill it with clouds. Filters -> Render -> Clouds -> Solid Noise -> Set Detail and the sizes to maximum.

5. Adjust the brightness of the cloud layer. Colors -> Brightness-Contrast -> Turn brightness to about 80.

6. Select the layer with your blurry coastline and move it above the layer with noise, then set the mode (Dropdown at the top of the layer window) to Multiply and merge the two layers by right clicking on the top one, which should be the blurry outline of your continent, and click "Merge Down". You should now be left with a single layer looking somewhat like the image below.

7. Go to Colors -> Threshold and adjust the sliders till you like the result. Keep in mind that if massive lakes appear inland where you don't want them, you can easily fill them manually afterwards.

8. Chose the Select by Color tool (finger pointing to red), select the black and press delete. You now have a layer of free floating islands.
9. Select the white, chose the Bucket Fill tool (paint bucket), tick "Fill whole selection" and fill it with green. Make a new layer below the islands and fill it with blue.

10. Now you can go over it manually, removing any small islands by erasing them or lakes by filling them with green. This is also where you will want to draw rivers large enough to be seen at the scale your map is representing. Small rivers only a few pixels wide will look messy by the end so better to save those for more regional maps, or simply ignore them. And if you do draw rivers, always remember, rivers join up on the way to the ocean, they don't split. As an optional step to keep organized, name your blue layer "Water" and green layer "Land".

Continent Border

These changes are very minor on their own, but adds to the clean look of the final product
11. Right click on your land layer -> Alpha to Selection. Now make a new layer and name it "Border". Make sure you still have the shape of the lands selected while having this new empty layer selected and go to Select -> Grow -> 1 px. Now that your selected is slightly larger than the land it is time to get rid of those hard edges. Go to Select -> Feather -> 3 px. Now fill the entire area with black.

12. Right click on your land layer again -> Alpha to Selection. This time we will do the inverse, so go to Select -> Shrink -> 1 px, followed by Select -> Feather -> 3 px. Now, making sure you have the Border layer selected, hit delete. Change the Opacity (Slider above the layers) of the border layer to 30.

13. Now to add some detail to the water. Make a new layer and call it "Shallow". Right click on your land layer -> Alpha to Selection. Grow your selection by 10 px, and then feather it by 50. Now fill the new layer with white, and move the layer to be below Land and Border, but above water. Change the layers opacity to about 25, to get a nice gradiant from water to land.

14. To add some height to the map, duplicate the entire land layer by right clicking it and hiting duplicate. Alpha to Selection on your copied land, and paint it completely black. Select the Move Tool and move the layer down about 3 pixels (just click the down arrow 3 times) and put the new layer behind your actual land and rename it "Shadow Cliffs". Then change its opacity to about 40 or so.

These follow two steps are somewhat optional and only really a benefit you if your map will be high resolution in the end. But its a nice touch regardless.
15. Now that you should know how to do most of the basics, I'll stop explaining every button press. So, Make a new layer, Alpha to Selection on Land again, Grow the selection by 3 and fill it with black, then shrink the selection again by 1 and delete. You should now have a thin black line running along the coast slightly out in the water.

16. Repeat the previous step on another new layer, but grow by 6 rather than 3. Now you should have 2 lines, each on a different layer. Change the opacity of these layers down to about 20 and 10, or whatever you feel looks right. Once you are happy with it, merge the two layers together and name it "Waves"

Weathering

17. Google "Parchment" and find one you like. Preferably get one that is as big or bigger than your map, otherwise you'll have to scale it up which might make it look terrible. Copy the image and paste it into GIMP. It should auto-create a layer called "Floating Selection", double click on the layer to rename it and call it "Parchment". I'll be using this
18. Go to Colors -> Hue-Saturation and turn the saturation all the way down so you get a grey piece of parchment. Then right click on the image, Layer -> Transparency -> Color to Alpha, and get rid of white. Lastly turn down the layers opacity till you like the look (I'll go to about 40)

Turning a Landmass into a Map

19. Create a new layer called Biomes and select the alpha of your land again. Using the softest brush with paint tool, start painting the land various shades of green, and painting in deserts and arctic areas.

20. Download these brushes by StarRaven on Deviantart.
The exact location to extract them to varies somewhat depending on your version of GIMP. The easiest way to find the correct folder is to rightclick on one of the brushes in the box below your list of layers and hit "Copy Brush Location". Copy that somewhere you can read it, or just directly into the address bar of a folder window and go to the "Brushes" folder. Extract the .abr file to here.
21. Right click the box with the brushes below the layers and click "Refresh Brushes". You should now see a bunch of new brushes with mountains and forrests.
22. Create new layers, one for trees and another for mountains, and start filling your continents as you see fit. Try to vary up the brushes to avoid too much obvious repeating.

23. Repeat the previous step for cities and other settlements. The icon you chose for each is completely up to you. Personally I like doing these in red to make them more visible and make them seem different to everything else so far that has just been part of nature. But if you prefer them black or some other color that doesn't look terrible with the terrain, that would work too.
Keep in mind that, if you are doing a continent or world sized map, you probably just want to mark the large or plot-significant settlements, since marking every village will take forever. Just keep in mind that in general there would be a village about every days ride between big settlements.

24. Find a suitably medieval font from a site like dafont.com or similar. I'll be using Cardinal. Now comes the tedious part of thinking up names for every city. As you think of names type them in a suitably sized font over the settlement, potentially varying depending on the size of the settlement. Don't worry about the text blending in with trees and mountains, we'll fix that in the next step.
You can also write names for landmasses, lakes, oceans etc. at this stage if you want to. Rotating text slightly to follow the flow of the terrain can look nice as well.

25. Make a new layer group (the folder icon below the list of layers) and move all the new text layers into it. Call it "Labels"
26. Create a new layer called "Label Border". Then Right click on our Labels layer group and do the familiar "Alpha to Selection". Make sure the new layer is below all your labels. Doesn't matter if it is inside or outside the layer group. Assuming you still have the alpha of the labels selected, Do Select -> Grow -> 1 px, then feather that by 5 px. Fill it all with white. The labels should now be easily distinguishable even over mountains and trees. You can turn down the opacity of this layer to make it less obvious, and if it still looks too gerrish for certain places, you can go over it manually and erase it since it is its own layer.
You can even do the same selection, but rather than fill it with white, simply select the mountains and tree layers and hit delete. Keep in mind you might want to back these layers up first though (Could also be done with layer masks but I don't want to make it too complicated)

27. Using the same method as with the parchment, you can now add things like a compass rose, a ruler for scale, or giant sea-monsters etc. Downloading additional brush packs could also help with this.

28. Optional and very time consuming step. If you want your mountains to pop more, you can go over them in a new layer right below the "Mountain" layer, and fill them in with grey and a second pass with white for the peaks.

29. Another optional step. To add some more infrastructure to your world, and show your players the common and largest routes between areas, you can mark roads with the common "dotted red line" by using the "Paths Tool" (Ink pen next to a white line). Make sure to create a new layer again, named something like "Routes". Click where you want the path to start, then click and hold where you want it to end, and drag the cursor to make the road curve slightly for a more natural look. Once you like the path, click "Stroke Path". To select the look of your path open the small "Line Style" menu.
Personally I use a custom style with evenly sized dashes and spaces, a line width of 3 and lower the layers opacity slightly, but the style is really up to you. I'd recommend using the same color you used for your settlement markers, but again, up to you. For longer paths around mountains etc, you'll want more waypoints in there, but you should be able to get the hang of the tool rather quickly.